Support MIPS rel6

Support MIPS rel6

Don't set mips2/3 for mips r6.  MIPS release 6 changes the encoding of
some instructions, including ll/sc.  Setting mips2/3 here produces the
binary with the old encoding (not compatible with mips r6).

This patch has no effect if gcc 4.9+ or clang 3.5+ unless
AO_DISABLE_GCC_ATOMICS is defined manually in CFLAGS.

* src/atomic_ops/sysdeps/gcc/mips.h [AO_DISABLE_GCC_ATOMICS
&& __mips_isa_rev>=6] (AO_MIPS_SET_ISA): Define to empty string
(instead of ".set mips2" or ".set mips3").