[Clang] Fix clang_rt tests when LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is ON

If clang is part of a toolchain that can locate clang_rt libraries
outside its resource directory and these are built with
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON then the tests would fail because
the library names don't have the arch suffix. This change makes the arch
suffix optional.

Differential Revision: https://reviews.llvm.org/D143666