[Driver] Enable multilib.yaml in the BareMetal ToolChain

The default location for multilib.yaml is lib/clang-runtimes, without
any target-specific suffix. This will allow multilibs for different
architectures to share a common include directory.

To avoid breaking the arm-execute-only.c CHECK-NO-EXECUTE-ONLY-ASM
test, add a ForMultilib argument to getARMTargetFeatures.

Since the presence of multilib.yaml can change the exact location of a
library, relax the baremetal.cpp test.

Differential Revision: https://reviews.llvm.org/D142986