[patch] declare __clear_cache

[Tim Northover]

Hi Rafael,

> While testing another patch I noticed that on my system (fedora on an
> arm chromebook) libgcc defines __clear_cache, but it is not declared
> in any header.

I thought builtins didn't need declarations? I can't provoke any
warning or error on a vaguely GCC-like compiler.

Though I do notice that I've declared it incorrectly on AArch64 (your
patch breaks self-hosting and the libgcc documentation is clear that
it's my fault).

Tim.