[Builtins] Mark mulsc3_test as UNSUPPORTED for armhf; NFC

The same test fails on clang-cmake-armv7-a15-full biuld bot
but passes the clang-cmake-thumbv7-a15-full.

For now, we mark it as UNSUPPORTED for armhf target.
Bug 32457 tracks it.

llvm-svn: 299005