MFC r311896
Remove unused __gnu_inline() attribute.

This was meant to be used by a future FORTIFY_SOURCE implementation.
Probably for good, FORTIFY_SOURCE and this particular GCCism were never
well supported by clang or other compilers. Furthermore, the technology
has long since been replaced by either static checkers, sanitizers, or
even just the strong stack protector that was enabled by default.

Drop __gnu_inline to avoid cluttering the headers.