Use LLVM as default toolchain for all PowerPC targets

This enables LLVM as the default compiler for powerpc, powerpc64, and
powerpcspe, as well as LLD as the default linker for powerpc64.

LLD is not yet ready for prime time for powerpc and powerpcspe, but work is
continuing on it.

Submitted by: alfredo.junior_eldorado.org.br
Relnotes: YES
Differential Revision: https://reviews.freebsd.org/D20378