Add support for a MIPS specific .MIPS.xhash section.

Add support for a MIPS specific .MIPS.xhash section.

  This patch is a reimplementation of [1] which was submitted in 2015 by
Neil Schellenberger. Copyright issue was sorted out [2] last year.
It proposed a new section (.gnu.xhash) and related dynamic tag
(DT_GNU_XHASH). The new section would be virtually identical to the
existing .gnu.hash except for the translation table (xlat) which would
contain correct MIPS .dynsym indexes corresponding to the hashvals in
chains. This is because MIPS ABI imposes a different ordering on the
dynsyms than the one expected by the .gnu.hash section. Another addition
would be a leading word (ngnusyms) which would contain the number of
entries in the translation table.

  In this patch, the new section name and dynamic tag are changed to
reflect the fact that the section should be treated as MIPS-specific
(.MIPS.xhash and DT_MIPS_XHASH).

  This patch addresses the alignment issue as reported in [3], which is
caused by the leading word added to the .MIPS.xhash section. Leading word
is removed in this patch, and the number of entries in the translation
table is now calculated using DT_MIPS_SYMTABNO dynamic tag (this is
addressed by the corresponding glibc patch).

  Suggestions on coding style in [4] were taken into account. Existing
GNU hash testcase was covered, and another one was added in the MIPS
part of the testsuite.

  The other major change is reserving MIPS ABI version 5 for .MIPS.xhash,
marking the need of support for .MIPS.xhash in the dynamic linker (again,
addressed in the corresponding glibc patch). This is something which I
am not sure of, especially after reading [5]. I am confused on whether
this ABI version is reserved for IFUNC, or it can be used for this
purpose.

Already mentioned glibc patch is submitted at:
https://sourceware.org/ml/libc-alpha/2019-06/msg00456.html

[1] https://sourceware.org/ml/binutils/2015-10/msg00057.html
[2] https://sourceware.org/ml/binutils/2018-03/msg00025.html
[3] https://sourceware.org/ml/binutils/2016-01/msg00006.html
[4] https://sourceware.org/ml/binutils/2016-02/msg00097.html
[5] https://sourceware.org/ml/libc-alpha/2016-12/msg00853.html

ld      * emulparams/elf32bmip.sh: Add .MIPS.xhash section.
        * emulparams/elf32bmipn32-defs.sh: Add .MIPS.xhash section.
        * emulparams/elf64bmip-defs.sh: Add .MIPS.xhash section.
        * emultempl/mipself.em: Remove mips_after_parse function.
        * testsuite/ld-elf/hash.d: Update comment.
        * testsuite/ld-mips-elf/hash1.d: New test.
        * testsuite/ld-mips-elf/hash1.s: Ditto.
        * testsuite/ld-mips-elf/hash1a.d: Remove.
        * testsuite/ld-mips-elf/hash1b.d: Ditto.
        * testsuite/ld-mips-elf/hash1c.d: Ditto
        * testsuite/ld-mips-elf/hash2.d: New test.
        * testsuite/ld-mips-elf/mips-elf.exp: New tests.
        * testsuite/ld-mips-elf/start.s: New test.

bfd     * elf-bfd.h (struct elf_backend_data): New members.
        * elflink.c (_bfd_elf_link_create_dynamic_sections): Create
        .gnu.hash section if necessary.
        (struct collect_gnu_hash_codes): New member.
        (elf_gnu_hash_process_symidx): New function name.
        (elf_renumber_gnu_hash_syms): Ignore local and undefined
        symbols. Record xlat location for every symbol which should have
        a .MIPS.xhash entry.
        (bfd_elf_size_dynamic_sections): Add DT_GNU_HASH dynamic tag to
        dynamic section if necessary.
        (GNU_HASH_SECTION_NAME): New define.
        (bfd_elf_size_dynsym_hash_dynstr): Get .MIPS.xhash section.
        Update the section size info.
        * elfxx-mips.c (struct mips_elf_hash_sort_data): New members.
        (struct mips_elf_link_hash_entry): New member.
        (mips_elf_link_hash_newfunc): Initialize .MIPS.xhash translation
        table location.
        (mips_elf_sort_hash_table): Initialize the pointer to the
        .MIPS.xhash section.
        (mips_elf_sort_hash_table_f): Populate the .MIPS.xhash
        translation table entry with the symbol dynindx.
        (_bfd_mips_elf_section_from_shdr): Add SHT_MIPS_XHASH.
        (_bfd_mips_elf_fake_sections): Initialize .MIPS.xhash section
        info.
        (_bfd_mips_elf_create_dynamic_sections): Create .MIPS.xhash
        section.
        (_bfd_mips_elf_size_dynamic_sections): Add DT_MIPS_XHASH tag to
        dynamic section.
        (_bfd_mips_elf_finish_synamic_sections): Add DT_MIPS_XHASH.
        (_bfd_mips_elf_final_write_processing): Set .MIPS.xhash section
        sh_link info.
        (_bfd_mips_elf_get_target_dtag): Get DT_MIPS_XHASH tag.
        (MIPS_LIBC_ABI_XHASH): New ABI version enum value.
        (_bfd_mips_post_process_headers): Mark the ABI version as
        MIPS_LIBC_ABI_XHASH if there exists a .MIPS.xhash section,
        but not a .hash section.
        (_bfd_mips_elf_record_xhash_symbol): New function. Record a
        position in the translation table, associated with the hash
        entry.
        * elfxx-mips.h (literal_reloc_p): Define
        elf_backend_record_xhash_symbol backend hook.
        * elfxx-target.h: Initialize elf_backend_record_xhash_symbol
        backend hook.

include * elf/mips.h (SHT_GNU_XHASH): New define.
        (DT_GNU_XHASH): New define.

binutils * readelf.c (get_mips_dynamic_type): Return MIPS_XHASH dynamic type.
        (get_mips_section_type_name): Return MI{S_XHASH name string.
        (dynamic_section_mips_val): Initialize the .MIPS.xhash dynamic
        info.
        (process_symbol_table): Initialize the .MIPS.xhash section
        pointer. Adjust the readelf output to support the new section.
        (process_object): Set the .MIPS.xhash dynamic info to zero.