gh-98178: syslog() is not thread-safe on macOS (GH-98213)

On macOS, fix a crash in syslog.syslog() in multi-threaded
applications. On macOS, the libc syslog() function is not
thread-safe, so syslog.syslog() no longer releases the GIL to call
it.
(cherry picked from commit d4b91663857e85eab1f309cacec4d27b5f6657ec)

Co-authored-by: Victor Stinner <vstinner@python.org>