Teach the Linux toolchain about more modern Gentoo installations of GCC
which add another wrinkle to the installation of the libstdc++ headers.

Add at least some basic testing of the weirdnesses of Gentoo's layout.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@189212 91177308-0d34-0410-b5e6-96231b3b80d8