kselftest/arm64: Support build of MTE tests with clang

kselftest/arm64: Support build of MTE tests with clang

The assembly portions of the MTE selftests need to be built with a
toolchain supporting MTE.  Since we support GCC versions that lack MTE
support we have logic to suppress build of these tests when using such a
toolchain but that logic is broken for LLVM=1 builds, it uses CC but CC
is only set for LLVM builds in libs.mk which needs to be included after
we have selected which test programs to build.

Since all supported LLVM versions support MTE we can simply assume MTE
support when LLVM is set.  This is not a thing of beauty but it does the
job.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230111-arm64-kselftest-clang-v1-5-89c69d377727@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>