Remove MIPS16 atomics using __sync_* (bug 17404).

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.