pixelflinger: use __builtin___clear_cache instead of cacheflush

cacheflush doesn't exist on LP64 any more, and gcc's
__builtin___clear_cache is better in every way.  Use it instead.

Change-Id: Ibbf6facbdefc15b6dda51d014e1c44fb7aa2b17d

Conflicts:
	libpixelflinger/codeflinger/CodeCache.cpp
	libpixelflinger/tests/arch-arm64/assembler/arm64_assembler_test.cpp