[musl] Always cancelasync as a boolean. NFC

This is how all the other code in musl already treats it.

Author: sbc100

system/lib/pthread/emscripten_futex_wait.c

@@ -131,7 +131,7 @@ int emscripten_futex_wait(volatile void *addr, uint32_t val, double max_wait_ms)
 
 #ifdef __EMSCRIPTEN_PTHREADS__
   pthread_t self = pthread_self();
-  bool cancelable = self->cancelasync == PTHREAD_CANCEL_ASYNCHRONOUS;
+  bool cancelable = self->cancelasync;
 #else
   bool cancelable = false;
 #endif

system/lib/pthread/pthread_create.c

@@ -309,7 +309,7 @@ void _emscripten_thread_exit(void* result) {
   assert(self);
 
   self->canceldisable = PTHREAD_CANCEL_DISABLE;
-  self->cancelasync = PTHREAD_CANCEL_DEFERRED;
+  self->cancelasync = 0;
   self->result = result;
 
   _emscripten_thread_mailbox_shutdown(self);