selftests/membarrier: skip unpermitted membarrier command test if preregistered by libc

On thread creation, Musl registers the private expedited memory barrier,
see pthread_create [1].  Thus, invoking the barrier command will no longer
be rejected by the kernel with EPERM.  The test checking this will fail. 
Check if the memory barrier command has been registered and skip the test
in this case.

Link: https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_create.c#n260 [1]
Link: https://lore.kernel.org/20260803124900.3328789-3-christian.gellermann@codasip.com
Signed-off-by: Chris Gellermann <christian.gellermann@codasip.com>
Tested-by: Michael Jeanson <mjeanson@efficios.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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