[PATCH] fix BZ 18116 - build failure on ppc64le: setcontext.S uses power6 mtfsf when not supported

[Martin Sebor]

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. The fix in the proposed patch introduces the .machine
"power6" directive unconditionaly, regardless of whether
_ARCH_PWR6 is defined.

Martin
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