[Bug testsuite/66734] Many MPX tests are skipped

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66734

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> ---
Since check_effective_target_mpx caches the result, if the first
MPX check fails, none of MPX tests will run.