MIPS: clear_cache, use _flush_cache instead of cacheflush

The cacheflush is only defined with __USE_MISC, which depends on _DEFAULT_SOURCE,
_GNU_SOURCE or _BSD_SOURCE, or _SVID_SOURCE.

If CC is called with -std=c11, these macros won't be defined, Let's use
_flush_cache, which is defined always.

Reviewed By: brad, jrtc27

Differential Revision: https://reviews.llvm.org/D156072