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: https://llvm.org/svn/llvm-project/cfe/trunk@184662 91177308-0d34-0410-b5e6-96231b3b80d8