runtime: Create more symlinks for ${PLATFORM}-dependent libdirs
Since glibc 2.26, the ${PLATFORM} for x86_64 can expand to "haswell" or "xeon_phi" in addition to "x86_64".
To make it clearer what's going on, stop pretending that library paths based on ${PLATFORM} have anything to do with multiarch tuples. Instead, create symlinks like /overrides/lib/platform-haswell -> x86_64-linux-gnu and /overrides/lib/platform-i686 -> i386-linux-gnu.
/cc @denittis
This is partly correctness (makes 64-bit VDPAU drivers available on post-Haswell CPUs with recent glibc) and partly clarity (making the scope of our VDPAU driver path hack more visible).