AArch64: correct definition of __clear_cache

According to libgcc document __clear_cache takes two char*
pointers. I suspect GCC's actual behaviour is more subtle than that,
but char* should clearly be preferred to void*.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181762 91177308-0d34-0410-b5e6-96231b3b80d8