[runloop] Avoid pthread_main_np on OpenBSD.

During runloop testing, log messages suggest that the runloop thinks the
main thread has exited and certain runloop operations don't actually
proceed. The flag for denoting when the main thread has exited is set in
the thread-specific data destructor `__CFTSDFinalize`.

This function detects whether the thread in question is the main thread
via `pthread_main_np`, and has been observed in debugging this problem
to return 1 on apparently non-main threads on OpenBSD. This obviously
will cause the main thread exited flag to be erroneously set when
non-main threads complete their work and dispose of thread-specific
data.

Instead, use the existing `_CFMainPThread` symbol set in
`__CFInitialize` to determine the main thread. I am not sure whether the
platform `pthread_main_np` is at fault here, but since this workaround
makes the tests pass, let's use it.

Author: 3405691582

Diff for: CoreFoundation/Base.subproj/CFPlatform.c

@@ -249,7 +249,11 @@ const char *_CFProcessPath(void) {
 
 #if TARGET_OS_MAC || TARGET_OS_WIN32 || TARGET_OS_BSD
 CF_CROSS_PLATFORM_EXPORT Boolean _CFIsMainThread(void) {
+#if defined(__OpenBSD__)
+    return pthread_equal(pthread_self(), _CFMainPThread) != 0;
+#else
     return pthread_main_np() == 1;
+#endif
 }
 #endif
 
@@ -774,7 +778,7 @@ static void __CFTSDFinalize(void *arg) {
 #if TARGET_OS_WASI
     __CFMainThreadHasExited = true;
 #else
-    if (pthread_main_np() == 1) {
+    if (_CFIsMainThread()) {
         // Important: we need to be sure that the only time we set this flag to true is when we actually can guarentee we ARE the main thread. 
         __CFMainThreadHasExited = true;
     }