[clang] Allow using BareMetal toolchain with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR compi...

[clang] Allow using BareMetal toolchain with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR compiler-rt

If you build compiler-rt with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR then
the library filename will be "libclang_rt.builtins.a" instead of
"libclang_rt.builtins-<ARCH>.a"

The ToolChain::getCompilerRT method uses the "libclang_rt.builtins.a"
name if it can find the file in the library directories. If it can't
then it falls back to using "libclang_rt.builtins-<ARCH>.a". This
change adds the library directory such that "libclang_rt.builtins.a"
can be found.

Differential Revision: https://reviews.llvm.org/D139822