Fix --disable-rpath and tests

This involved a few changes to the local build system:

* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

This change was spliced out of #14832 to consist of just the fixes to running
tests without an rpath setting embedded in executables.