mk: Get "make check" passing with --disable-rpath

This involves passing through LD_LIBRARY_PATH through more places, specifically
in the compiletest, run-make, and doctest runners.
auto merge of #12530 : alexcrichton/rust/make-check-no-rpath, r=brson

This involves passing through LD_LIBRARY_PATH through more places, specifically
in the compiletest, run-make, and doctest runners.