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

[luporl]

Summary

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.

Test Plan

A full build and install was performed.
This confirmed that ld.bfd was properly installed and that /usr/bin/ld links to ld.lld when lld is used, as expected.

[jhibbits]

This looks fine to me. Hoping we can obliterate GNU ld before too long, even on 32-bit.