gc: fix missing pthread registration causing unknown thread error

- Use `GC_pthread_create` instead of `pthread_create` when GC is
  enabled.

c/pthread/_pthread/pthread_gc.c

@@ -0,0 +1,11 @@
+#define GC_THREADS
+#include <gc.h>
+#include <pthread.h>
+
+int llgoPthreadCreate(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg) {
+    return GC_pthread_create(thread, attr, start_routine, arg);
+}
+
+int llgoPthreadJoin(pthread_t thread, void **retval) {
+    return GC_pthread_join(thread, retval);
+}

c/pthread/_pthread/pthread_nogc.c

@@ -0,0 +1,9 @@
+#include <pthread.h>
+
+int llgoPthreadCreate(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg) {
+    return pthread_create(thread, attr, start_routine, arg);
+}
+
+int llgoPthreadJoin(pthread_t thread, void **retval) {
+    return pthread_join(thread, retval);
+}