Fix __clear_cache on ARM.

Current gcc's produce an error if __clear_cache is anything but

__clear_cache(char *a, char *b);

It looks like we had just implemented a gcc bug that is now fixed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181784 91177308-0d34-0410-b5e6-96231b3b80d8