cmake/compiler: Limit warning flag usage to compatible toolchains

New C++ versions have deprecated "register" variables and restricted
"volatile" semantics, so new gcc's will emit warnings when they see
that syntax.  Zephyr uses both in our C headers (though we should
probably get rid of register and unify with C++'s volatile model), so
we're disabling the resulting warnings.

But OLD gcc variants (like xcc, sigh) don't understand new -Wvolatile
and -Wregister on the command line, so they get confused.  Limit the
uses to the standard versions for which gcc would emit warnigns; xcc
doesn't support those anyway.

Signed-off-by: Andy Ross <andyross@google.com>