Darwin(ish): enable embedded compiler-rt builds on Darwin.
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@194878 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang