LoongArch: Add R_LARCH_64_PCREL relocation support

LoongArch: Add R_LARCH_64_PCREL relocation support

  Gas defaults to emit R_LARCH_ADD64/R_LARCH_SUB64 unless explcitly declared
  to emit R_LARCH_64_PCREL.

  The LoongArch ABI at here:
    https://github.com/loongson/la-abi-specs/blob/release/la-abi.adoc

bfd/ChangeLog:

	* bfd-in2.h (not): Add R_LARCH_64_PCREL
	* elfnn-loongarch.c (perform_relocation): Likewise.
	* elfxx-loongarch.c: Likewise.
	* libbfd.h: Likewise.
	* reloc.c: Likewise.

gas/ChangeLog:

	* config/tc-loongarch.c (loongarch_args_parser_can_match_arg_helper):
	(md_apply_fix): Add R_LARCH_64_PCREL.
	* testsuite/gas/loongarch/64_pcrel.d: New test.
	* testsuite/gas/loongarch/64_pcrel.s: New test.

include/ChangeLog:

	* elf/loongarch.h (RELOC_NUMBER): Add R_LARCH_64_PCREL.

ld/ChangeLog:

	* testsuite/ld-loongarch-elf/ld-loongarch-elf.exp: Add test.
	* testsuite/ld-loongarch-elf/64_pcrel.d: New test.
	* testsuite/ld-loongarch-elf/64_pcrel.s: New test.