pthread_get_stacksize_np: Cache dlsym() result.

This avoids repeatedly looking up the symbol for the OS call.

Also cleans up indentation.

TESTED:
Passes tests on all platforms.

src/pthread_get_stacksize_np.c

@@ -59,12 +59,16 @@ size_t pthread_get_stacksize_np(pthread_t t) {
         }
     } else {
         /* bug only affects main thread */
-	  size_t (*real_pthread_get_stacksize_np)(pthread_t);
-	  real_pthread_get_stacksize_np = dlsym(RTLD_NEXT, "pthread_get_stacksize_np");
-	  if (real_pthread_get_stacksize_np == NULL) {
-	       exit(EXIT_FAILURE);
+        static size_t (*os_pthread_get_stacksize_np)(pthread_t);
+        if (!os_pthread_get_stacksize_np) {
+            os_pthread_get_stacksize_np =
+                dlsym(RTLD_NEXT, "pthread_get_stacksize_np");
+            /* Something's badly broken if this fails */
+            if (!os_pthread_get_stacksize_np) {
+                abort();
+            }
         }
-	  return real_pthread_get_stacksize_np(t);
+        return (*os_pthread_get_stacksize_np)(t);
     }
 }