bfd: allow negative offsets to _GLOBAL_OFFSET_TABLE_ in elf64 SPARC

bfd: allow negative offsets to _GLOBAL_OFFSET_TABLE_ in elf64 SPARC

The code compiled with the -fpic model in SPARC uses 13-bit signed
immediate PC-relative loads to fetch entries from the GOT table.  In
theory this would allow using a GOT table (.got section) containing up
to 1024 entries in elf32 or 512 entries in elf64.

However, in elf64 sparc GNU targets _GLOBAL_OFFSET_TABLE_ is always
placed at the beginning of the .got section, making it impossible to use
negative offsets.  This limits the usage of -fpic to GOT tables
containing a maximum of 257 entries in elf64.

This patch activates an optimization that is already used in sparc-elf32
also in sparc-elf64, that sets _GLOBAL_OFFSET_TABLE_ to point 0x1000
into the .got section if the section size is bigger than 0x1000.

2016-09-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* elfxx-sparc.c (_bfd_sparc_elf_size_dynamic_sections): Allow
	negative offsets to _GLOBAL_OFFSET_TABLE_ if the .got section is
	bigger than 0x1000 bytes.