Add EXPERIMENTAL --rtlib=compiler-rt to GNU Clang

This commit is not strictly correct nor accounts for all uses (shared
objects, for example), but it allows one to test the compiler-rt library
on GNU targets.

Using this patch to run the test-suite has already shown me problems
on ARM. Since this is a Darwin-only flag, nobody is using it, so it
shouldn't be a problem.

I will need extension to deal with the shared cases, but since we're
not compiling libclang_rt.so, that's not yet applicable. Many other
problems will have to be fixed first in compiler-rt (such as removing
the 'arch' name from it and making it trully multi-arch, moving it to
the default lib directory, make both .a and .so variants, etc).