drop use of pthread_once in mutexattr kernel support tests

this makes the code slightly smaller and eliminates these functions
from relevance to possible future changes to multithreaded fork.

the barrier of a_store isn't technically needed here, but a_store is
used anyway for internal consistency of the memory model.

src/thread/pthread_mutexattr_setprotocol.c

@@ -1,24 +1,23 @@
 #include "pthread_impl.h"
 #include "syscall.h"
 
-static pthread_once_t check_pi_once;
-static int check_pi_result;
-
-static void check_pi()
-{
-	volatile int lk = 0;
-	check_pi_result = -__syscall(SYS_futex, &lk, FUTEX_LOCK_PI, 0, 0);
-}
+static volatile int check_pi_result = -1;
 
 int pthread_mutexattr_setprotocol(pthread_mutexattr_t *a, int protocol)
 {
+	int r;
 	switch (protocol) {
 	case PTHREAD_PRIO_NONE:
 		a->__attr &= ~8;
 		return 0;
 	case PTHREAD_PRIO_INHERIT:
-		pthread_once(&check_pi_once, check_pi);
-		if (check_pi_result) return check_pi_result;
+		r = check_pi_result;
+		if (r < 0) {
+			volatile int lk = 0;
+			r = -__syscall(SYS_futex, &lk, FUTEX_LOCK_PI, 0, 0);
+			a_store(&check_pi_result, r);
+		}
+		if (r) return r;
 		a->__attr |= 8;
 		return 0;
 	case PTHREAD_PRIO_PROTECT:

src/thread/pthread_mutexattr_setrobust.c

@@ -1,22 +1,20 @@
 #include "pthread_impl.h"
 #include "syscall.h"
 
-static pthread_once_t check_robust_once;
-static int check_robust_result;
-
-static void check_robust()
-{
-	void *p;
-	size_t l;
-	check_robust_result = -__syscall(SYS_get_robust_list, 0, &p, &l);
-}
+static volatile int check_robust_result = -1;
 
 int pthread_mutexattr_setrobust(pthread_mutexattr_t *a, int robust)
 {
 	if (robust > 1U) return EINVAL;
 	if (robust) {
-		pthread_once(&check_robust_once, check_robust);
-		if (check_robust_result) return check_robust_result;
+		int r = check_robust_result;
+		if (r < 0) {
+			void *p;
+			size_t l;
+			r = -__syscall(SYS_get_robust_list, 0, &p, &l);
+			a_store(&check_robust_result, r);
+		}
+		if (r) return r;
 		a->__attr |= 4;
 		return 0;
 	}