Darwin(ish): enable embedded compiler-rt builds on Darwin.

This builds extra versions of compiler-rt targeting various unhosted targets.
Only built on Darwin since even though they're not iOS or OSX, they share some
quirks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194878 91177308-0d34-0410-b5e6-96231b3b80d8