libpixelflinger: compile with GCC instead of LLVM clang

 * Upstream changes from AOSP master implement __builtin___clear_cache
   instead of the old arch dependant cacheflush
 * Original AOSP master commit: https://android.googlesource.com/platform/system/core/+/32ea4a895c4c914f0347dd1e943bca0f474a9050%5E%21/#F1