[Driver] Fix rpath for compiler-rt

The compiler-rt library path can be either {resource_dir}/lib/{triple}
or {resource_dir}/lib/{OS}/{arch} depending on whether
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default is ON or OFF.
Currently, the rpath added by -rtlib-add-rpath only adds
the latter. This patch checks both and adds the one that exists.

Reviewed by: Fangrui Song

Differential Revision: https://reviews.llvm.org/D146686