Really fix the declaration of __clear_cache.

When I tested gcc's behaviour before, I forgot the extern "C", so it
would warn when the types *did* match.

So in the end
* __clear_cache takes two void pointers.
* aarch64 was correct before.
* libgcc's manual is wrong.
* this patch fixes arm.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181810 91177308-0d34-0410-b5e6-96231b3b80d8