MachO: add dummy armv6m architecture directory so clang_macho_embedded builds things.

Without some mention of armv6m in a subdirectory of builtins, the make code
doesn't even know that armv6m exists and is something it should be looking for
in the platform-specific Makefiles. This means that none of the functions
listed actually get built and we end up with an almost entirely empty
libclang_rt.a for armv6m.

Unfortunately, the assembly code in the usual arm directory has no hope of
running on armv6m, which only supports Thumb-1 (not even ARM mode), so adding
it there won't work. Realistically, we probably *will* want to put any
optimised versions in a separate directory, so creating it now is harmless.

rdar://problem/17613576