Now that multiple prefixes are much cheaper to search for GCC

installations, support them when installed directly under the system
root ('/lib/gcc/...' essentially).

With this, Clang can correctly detect and use a cross-compiling GCC
installation within a system root and use it.

Again, test cases will be coming in later commits, as I'm going to write
a few test cases that exercise nearly all of this logic.

llvm-svn: 141121