This is a follow up to r361432, changing the layout of per-target
runtimes to more closely resemble multiarch. While before, we used
the following layout:
<resource_dir>/<target>/lib/libclang_rt.<runtime>.<extension>
Now we use the following layout:
<resource_dir>/lib/<target>/libclang_rt.<runtime>.<extension>
This also more closely resembles the existing "non-per-target" layout:
<resource_dir>/lib/<os>/libclang_rt.<runtime>-<arch>.<extension>
This change will enable further simplification of the driver logic
in follow up changes.