[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