Now that multiple prefixes are much cheaper to search for GCC

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141121 91177308-0d34-0410-b5e6-96231b3b80d8