Add and tighten up tests for cross compiling tool selection. This now verifies that we run the assembler and linker in the correct mode, and that we can successfully use a bi-arch variant of a GCC installation in a generic cross compilation invocation of Clang.
Add and tighten up tests for cross compiling tool selection. This now
verifies that we run the assembler and linker in the correct mode, and
that we can successfully use a bi-arch variant of a GCC installation in
a generic cross compilation invocation of Clang.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@184662 91177308-0d34-0410-b5e6-96231b3b80d8

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