MIPS16 atomics used __sync_* with GCC before 4.7, which as noted in
bug 17404 is missing the required barrier semantics for
atomic_exchange_rel. This patch removes the code in question as dead
now GCC before 4.7 is no longer supported for building glibc.
Sanity tested with builds for MIPS.
[BZ #17404]
* sysdeps/mips/atomic-machine.h
[__GNUC_PREREQ (4, 8) || (__mips16 && __GNUC_PREREQ (4, 7))]:
Change conditional to [__GNUC_PREREQ (4, 8) || __mips16].
[__mips16 && !__GNUC_PREREQ (4, 7)]: Remove conditional code.
