kselftest/arm64: Support FORCE_TARGETS

kselftest/arm64: Support FORCE_TARGETS

The top level kselftest Makefile supports an option FORCE_TARGETS which
causes any failures during the build to be propagated to the exit status
of the top level make, useful during build testing. Currently the recursion
done by the arm64 selftests ignores this option, meaning arm64 failures are
not reported via this mechanism. Add the logic to implement FORCE_TARGETS
so that it works for arm64.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Will Deacon <will@kernel.org>