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
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