powerpc: setcontext.S uses power6 mtfsf when not supported [BZ #18116]

powerpc: setcontext.S uses power6 mtfsf when not supported [BZ #18116]

The attached patch fixes a glibc build failure with gcc 5 on powerpc64le
caused by a recent change in gcc where the compiler defines the
_ARCH_PWR6 macro when processing assembly files but doesn't invoke the
assembler in the corresponding machine mode (unless it has been
explicitly configured to target POWER 6 or later).  A bug had been filed
with gcc for this (65341) but was closed as won't fix. Glibc relies on
the _ARCH_PWR6 macro in a few .S files to make use of Power ISA 2.5
instructions (specifically, the four-argument form of the mtfsf insn).
A similar problem had occurred in the past (bug 10118) but the fix that
was committed for it didn't anticipate this new problem.