Various modifications to debug and/or fix `make check-stage1`

This is a series of modifications to help me debug some `run-make` test failures.

They mainly either:
* add instrumentation to aid debugging of linkage errors,
* introduce the ability to override internal settings within rustc, or
* fine tune some things in the Makefile where we are telling binaries to use a host-oriented path for finding dynamic libraries, when it should be feeding the binaries a *target-oriented* path for dynamic libraries.

r? @alexcrichton