[Driver] Change layout of per-target runtimes to resemble multiarch

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>.<ext>

Now we use the following layout:

[RESOURCE_DIR]/lib/<target>/libclang_rt.<runtime>.<ext>

This also more closely resembles the existing "non-per-target" layout:

[RESOURCE_DIR]/lib/<os>/libclang_rt.<runtime>-<arch>.<ext>

This change will enable further simplification of the driver logic
in follow up changes.

Differential Revision: https://reviews.llvm.org/D62469

llvm-svn: 361784