lib: posix: pthread_key: use spinlock instead of semaphore
lib: posix: pthread_key: use spinlock instead of semaphore

None of the operations that `pthread_key_sem` protected were
blocking, so simply make it a spinlock.

Also made the lock static.

Signed-off-by: Chris Friedt <b335fc22cf1358c7e834a5788f93b9f24396b889@meta.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr