[libc][pthread] fix -Wmissing-field-initializers (#126314)
Fixes:
llvm-project/libc/test/integration/src/pthread/pthread_rwlock_test.cpp:59:29:
warning: missing field '__preference' initializer
[-Wmissing-field-initializers]
59 | pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
| ^
Also, add a test that demonstrates the same issue for
PTHREAD_MUTEX_INITIALIZER, and fix that, too.
PTHREAD_ONCE_INIT does not have this issue and does have test coverage.
