[PowerPC64LE] Fix platform definitions for powerpc64le-*-freebsd*

Teach clang that powerpc64le-*-freebsd* is a valid triple.

This is already in upstream clang, but was too late for llvm 11.0.0.
Apply it directly for now, until it can be backported to llvm 11.0.1.

See upstream https://reviews.llvm.org/D73425 for details.

Reviewed by: dim (upstream version), emaste
Approved by: emaste
Differential Revision: https://reviews.freebsd.org/D26400