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