use bss instead of mmap for main thread's pthread thread-specific data

this simplifies code and removes a failure case

src/thread/pthread_key_create.c

@@ -1,6 +1,7 @@
 #include "pthread_impl.h"
 
 const size_t __pthread_tsd_size = sizeof(void *) * PTHREAD_KEYS_MAX;
+void *__pthread_tsd_main[PTHREAD_KEYS_MAX] = { 0 };
 
 static void nodtor(void *dummy)
 {