auto merge of #11750 : bnoordhuis/rust/follow-rustc-symlink, r=thestinger

Before this commit, rustc looked in `dirname $0`/../lib for libraries
but that doesn't work when rustc is invoked through a symlink.

This commit makes rustc look in `dirname $(readlink $0)`/../lib, i.e.
it first canonicalizes the symlink before walking up the directory tree.

Fixes #3632.