[PowerPC64] Use ld.bfd to build LIB32 and STAND - when using llvm

Summary:
This patch is to support ongoing work for replacing "GCC/BFD" by "CLANG/LLD" on
target PowerPC64 [1], by proposing a way to specify and/or locate a secondary
ld.bfd linker.
This is necessary as LLD currently doesn't support PowerPC 32 bits, so we keep
using BFD for the 32 bit stuff on PowePC64(LIB32 compatibility and
STAND/slof/loader.)

• creates LD_BFD variable pointing to ld.bfd
• use LD_BFD as linker for LIB32/compat
• Default behavior for other platforms aren't changed.

[1] https://wiki.freebsd.org/powerpc/llvm-elfv2

Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D20261