[PPC] Enable build/install of ld.bfd on base

When using LLVM+clang+lld on PowerPC64, ld.bfd is also needed,
to link 32-bit binaries correctly, as lld support for 32-bit is
still minimal.

This change enables it to be built and installed when lld is used.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20259