[PowerPC] Support powerpcle target in Clang [3/5]

Add powerpcle support to clang.

For FreeBSD, assume a freestanding environment for now, as we only need it in the first place to build loader, which runs in the OpenFirmware environment instead of the FreeBSD environment.

For Linux, recognize glibc and musl environments to match current usage in Void Linux PPC.

Adjust driver to match current binutils behavior regarding machine naming.

Adjust and expand tests.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D93919