Allow clang to build __clear_cache on ARM.

__clear_cache is special. It needs no signature, but is a real function in
compiler_rt or libgcc.

Patch by Andrew Turner.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183926 91177308-0d34-0410-b5e6-96231b3b80d8