[BFD, LD, AArch64, 3/3] Add --pac-plt to enable PLTs protected with PAC.

[BFD, LD, AArch64, 3/3] Add --pac-plt to enable PLTs protected with PAC.

This is part of the patch series to add support for BTI and
PAC in AArch64 linker.

1) This patch adds new definitions of PAC enabled PLTs
and both BTI and PAC enabled PLTs.
2) It also defines the new dynamic tag DT_AARCH64_PAC_PLT
for the PAC enabled PLTs.
3) This patch adds a new ld command line option: --pac-plt.
In the presence of this option, the linker uses the PAC
enabled PLTs and marks with DT_AARCH64_PAC_PLT.
4) In case both BTI and PAC are enabled the linker should
pick PLTs enabled with both and also use dynamic tags for both.
All these are made according to the new AArch64 ELF ABI
https://developer.arm.com/docs/ihi0056/latest/elf-for-the-arm-64-bit-architecture-aarch64-abi-2018q4

*** bfd/ChangeLog ***

2019-03-13  Sudakshina Das  <sudi.das@arm.com>

	* elfnn-aarch64.c (PLT_PAC_ENTRY_SIZE, PLT_PAC_SMALL_ENTRY_SIZE): New.
	(PLT_BTI_PAC_ENTRY_SIZE, PLT_BTI_PAC_SMALL_ENTRY_SIZE): New.
	(setup_plt_values): Account for PAC or PAC and BTI enabled PLTs.
	(elfNN_aarch64_size_dynamic_sections): Add checks for PLT_BTI_PAC
	and PLT_PAC_PLT.
	(elfNN_aarch64_finish_dynamic_sections): Account for PLT_BTI_PAC.
	(get_plt_type): Add case for DT_AARCH64_PAC_PLT.
	(elfNN_aarch64_plt_sym_val): Add cases for PLT_BTI_PAC and PLT_PAC.

*** binutils/ChangeLog ***

2019-03-13  Sudakshina Das  <sudi.das@arm.com>

	* readelf.c (get_aarch64_dynamic_type): Add case for
	DT_AARCH64_PAC_PLT.
	(dynamic_section_aarch64_val): Likewise.

*** include/ChangeLog ***

2019-03-13  Sudakshina Das  <sudi.das@arm.com>

	* elf/aarch64.h (DT_AARCH64_PAC_PLT): New.

*** ld/ChangeLog ***

2019-03-13  Sudakshina Das  <sudi.das@arm.com>

	* NEWS: Document --pac-plt.
	* emultempl/aarch64elf.em (OPTION_PAC_PLT): New.
	(PARSE_AND_LIST_LONGOPTS, PARSE_AND_LIST_OPTIONS): Add pac-plt.
	(PARSE_AND_LIST_ARGS_CASES): Handle OPTION_PAC_PLT.
	* testsuite/ld-aarch64/aarch64-elf.exp: Add the following tests.
	* testsuite/ld-aarch64/bti-pac-plt-1.d: New test.
	* testsuite/ld-aarch64/bti-pac-plt-2.d: New test.
	* testsuite/ld-aarch64/pac-plt-1.d: New test.
	* testsuite/ld-aarch64/pac-plt-2.d: New test.
	* testsuite/ld-aarch64/bti-plt-1.s: Add .ifndef directive.