Consider adding the absolute CFG_LIBDIR to the search paths for rustc

[alexcrichton]

In #11045, we're probably going to start always having an absolute path for CFG_LIBDIR. This means that librustc has one extra location to start search for by default for the target libraries. This libdir is essentially the same thing as self_exe_path().pop() for rustc, but as #11063 noted other usage of librustc is hamstrung by having to specify this directory.

[steveklabnik]

Triage: unclear what the status is of this today.

[alexcrichton]

I think we've got our story pretty square here today, so closing.