add test for bug in pthread_get_stacksize_np

see rust-lang/rust#43347
see https://trac.macports.org/ticket/68015

test/test_pthread_get_stacksize_np.c

@@ -0,0 +1,37 @@
+#include <pthread.h>
+#include <assert.h>
+#include <stdio.h>
+#include <stddef.h>
+
+#ifdef __arm64__
+#define kTooSmallStackSize 0x83000 /* default stack size of non-main thread on Apple silicon */
+#else
+#define kTooSmallStackSize 0x80000 /* from LLVM: 1 << 19, default stack size of non-main thread */
+#endif
+
+void *thread_function(void *arg) {
+     assert( !pthread_main_np() && "Unknown error, thread should *not* be main." );
+     return NULL;
+}
+
+int main()
+{
+     pthread_t thread;
+     pthread_t thread_detached;
+
+     assert( !pthread_create(&thread, NULL, thread_function, NULL) && "Unknown error, cannot create thread" );
+     assert( (pthread_get_stacksize_np(thread) == kTooSmallStackSize) && "Stack for non-main thread is not the default value" );
+     printf("Stack size for non-main thread seems to be ok\n");
+     pthread_join(thread, NULL);
+
+     assert( !pthread_create(&thread_detached, NULL, thread_function, NULL) && "Unknown error, cannot create thread" );
+     assert( !pthread_detach(thread_detached) && "Unknown error, cannot detach thread" );
+     assert( (pthread_get_stacksize_np(thread_detached) == kTooSmallStackSize) && "Stack for detached non-main thread is not the default value" );
+     printf("Stack size for detached detached non-main thread seems to be ok\n");
+
+     assert( pthread_main_np() && "Unknown error, thread should be main." );
+     assert( (pthread_get_stacksize_np(pthread_self()) > kTooSmallStackSize) && "Stack for main thread is too small" );
+     printf("Stack size for main thread seems to be ok\n");
+
+     return 0;
+}