Implement pthread_getguardsize() and pthread_setguardsize().
pthread_getguardsize() and pthread_setguardsize() are required
by the POSIX_THREADS_EXT Option Group as detailed in Section
E.1 of IEEE-1003.1-2017. However, they were formerly part of
XSI_THREADS_EXT.
The XSI_THREADS_EXT Option Group was required for PSE51, PSE52,
PSE53, and PSE54 conformance.
Signed-off-by: Christopher Friedt <cfriedt@meta.com>
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