Update .MIPS.abiflags to support MIPS R6

Backport from trunk.

bfd/

	* elfxx-mips.c (update_mips_abiflags_isa): Add E_MIPS_ARCH_32R6
	and E_MIPS_ARCH_64R6 support.

ld/testsuite/

	* ld-mips-elf/abiflags-strip10-ph.d: New file.
	* ld-mips-elf/mips-eld.exp: Run the new test.

gas/

	* config/tc-mips.c (mips_elf_final_processing): Add INSN_ISA32R6
	and INSN_ISA64R6 support.

gas/testsuite/

	* gas/mips/elf_arch_mips32r6.d: New file.
	* gas/mips/elf_arch_mips64r6.d: New file.
	* gas/mips/mips.exp: Run the new tests.
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