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

 #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)
 {

src/thread/pthread_self.c

@@ -3,8 +3,8 @@
 static struct pthread main_thread;
 
 /* pthread_key_create.c overrides this */
-static const size_t dummy = 0;
-weak_alias(dummy, __pthread_tsd_size);
+static const void *dummy[1] = { 0 };
+weak_alias(dummy, __pthread_tsd_main);
 
 #undef errno
 static int *errno_location()
@@ -14,13 +14,7 @@ static int *errno_location()
 
 static int init_main_thread()
 {
-	void *tsd = 0;
-	if (__pthread_tsd_size) {
-		tsd = mmap(0, __pthread_tsd_size, PROT_READ|PROT_WRITE,
-			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
-		if (tsd == MAP_FAILED) return -1;
-	}
-	main_thread.tsd = tsd;
+	main_thread.tsd = (void **)__pthread_tsd_main;
 	main_thread.self = &main_thread;
 	if (__set_thread_area(&main_thread) < 0)
 		return -1;