Add hooking functionality

This allows us to hook chosen functions and do interesting things there (in
particular: reentrancy checking).

test/unit/tsd.c

@@ -79,6 +79,7 @@ thd_start(void *arg) {
 }
 
 TEST_BEGIN(test_tsd_main_thread) {
+	test_skip_if(test_is_reentrant());
 	thd_start((void *)(uintptr_t)0xa5f3e329);
 }
 TEST_END