[PPC] Fix platform definitions when compiling FreeBSD powerpc64 as LE

As a prerequisite to doing experimental buids of pieces of FreeBSD PowerPC64 as little-endian, allow actually targeting it.

This is needed so basic platform definitions are pulled in. Without it, the compiler will only run freestanding.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D73425