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.
Fix __clear_cache declaration.

This fixes the build with gcc in gnu++98 and gnu++11 mode.