Add multithreaded test of sem_getvalue

Test coverage of sem_getvalue is fairly limited.  Add a test that runs
it on threads on each CPU.  For this purpose I adapted
tst-skeleton-thread-affinity.c; it didn't seem very suitable to use
as-is or include directly in a different test doing things per-CPU,
but did seem a suitable starting point (thus sharing
tst-skeleton-affinity.c) for such testing.

Tested for x86_64.

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