mk: Fix rpath on cross compile builds

After removing absolute rpaths, cross compile builds (notably the nightly
builders) broke. This is because the RPATH was pointing at an empty directory
because only the rustc binary is copied over, not all of the target libraries.
This modifies the cross compile logic to fixup the rpath of the stage0
cross-compiled rustc to point to where it came from.
auto merge of #13462 : alexcrichton/rust/fix-cross-rpath, r=brson

After removing absolute rpaths, cross compile builds (notably the nightly
builders) broke. This is because the RPATH was pointing at an empty directory
because only the rustc binary is copied over, not all of the target libraries.
This modifies the cross compile logic to fixup the rpath of the stage0
cross-compiled rustc to point to where it came from.