selftests, x86: Rework x86 target architecture detection

We currently fail to build on a non-multilib x86_64 target.  We
print a helpful error, but it's nicer to allow the build to succeed.
Fix it and improve cross-compilation support by detecting
architecture support directly and building only the relevant tests.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>