Switch C compiler used to compile GHC to base Clang.

This removes build dependency on gcc and runtime dependency on gcc's runtime libraries.

Big thanks to Gleb for working on this.

PR: 225185
Submitted by: Gleb Popov <6yearold@gmail.com>
Exp-run by: antoine
Reviewed by: pgj
Differential Revision: https://reviews.freebsd.org/D12043