Add support for Mips32r6 and Mips64r6.

Add support for Mips32r6 and Mips64r6.

elfcpp/
	* mips.h (R_MIPS_PC21_S2, R_MIPS_PC26_S2, R_MIPS_PC18_S3,
	R_MIPS_PC19_S2, R_MIPS_PCHI16, R_MIPS_PCLO16): New enums for
	Mips32r6 and Mips64r6 relocations.
	(r6_isa): New function.
gold/
	* mips.cc (relocation_needs_la25_stub): Add support for relocs:
	R_MIPS_PC21_S2 and R_MIPS_PC26_S2.
	(hi16_reloc): Add support for R_MIPS_PCHI16 relocation.
	(is_matching_lo16_reloc): Likewise.
	(lo16_reloc): Add support for R_MIPS_PCLO16 relocation.
	(Mips_output_data_plt::plt_entry_r6): New static data member for
	R6 PLT entry.
	(Target_mips::is_output_r6): New method.
	(Target_mips::Mips_mach): Add new enum constants.
	(Mips_relocate_functions::Status): Likewise.
	(Mips_relocate_functions::pchi16_relocs): New static data member.
	(Mips_relocate_functions::relpc21): New method.
	(Mips_relocate_functions::relpc26): Likewise.
	(Mips_relocate_functions::relpc18): Likewise.
	(Mips_relocate_functions::relpc19): Likewise.
	(Mips_relocate_functions::relpchi16): Likewise.
	(Mips_relocate_functions::do_relpchi16): Likewise.
	(Mips_relocate_functions::relpclo16): Likewise.
	(Mips_output_data_plt::do_write): Add support for Mips r6 plt
	entry.
	(Target_mips::mips_32bit_flags): Add E_MIPS_ARCH_32R6 support.
	(Target_mips::elf_mips_mach): Add E_MIPS_ARCH_32R6 and
	E_MIPS_ARCH_64R6 support.
	(Target_mips::update_abiflags_isa): Likewise.
	(mips_get_size_for_reloc): Add support for relocs: R_MIPS_PCHI16,
	R_MIPS_PCLO16, R_MIPS_PC21_S2, R_MIPS_PC26_S2, R_MIPS_PC18_S3 and
	R_MIPS_PC19_S2.
	(Target_mips::Scan::local): Add support for relocs: R_MIPS_PCHI16
	and R_MIPS_PCLO16.
	(Target_mips::Scan::global): Add support for relocs:
	R_MIPS_PC21_S2 and R_MIPS_PC26_S2.
	(Target_mips::Relocate::relocate): Call functions for resolving
	Mips32r6 and Mips64r6 relocations, and print error message for
	STATUS_PCREL_UNALIGNED.
	(Target_mips::Scan::get_reference_flags): Add support for relocs:
	R_MIPS_PCHI16, R_MIPS_PCLO16, R_MIPS_PC21_S2, R_MIPS_PC26_S2,
	R_MIPS_PC18_S3 and R_MIPS_PC19_S2.
	(Target_mips::elf_mips_mach_name): Add E_MIPS_ARCH_32R6 and
	E_MIPS_ARCH_64R6 support.