Add arch-specific directory to search path

Summary:

This change adds an arch-specific subdirectory in <ResourceDir>/lib/<OS>
to the linker search path.  This path also gets added as '-rpath' for
native compilation if a runtime is linked in as a shared object.  This
allows arch-specific libraries to be installed alongside clang.

Reviewers: danalbert, cbergstrom, javed.absar

Subscribers: srhines

Differential Revision: https://reviews.llvm.org/D30015

llvm-svn: 296927