Simplify TLS and reduce startup latency

This change simplifies the thread-local storage support code. On Windows
and Mac OS X the startup latency of __enable_tls() has been reduced from
30ms to 1ms. On Windows, TLS memory accesses will now go much faster due
to better self-modifying code that prevents a function call and acquires
our thread information block pointer in a single instruction.

libc/sysv/errno_location.greg.c

@@ -28,5 +28,5 @@
  */
 privileged nocallersavedregisters errno_t *(__errno_location)(void) {
   if (!__tls_enabled) return &__errno;
-  return (errno_t *)(__get_tls_inline() + 0x3c);
+  return (errno_t *)(__get_tls_privileged() + 0x3c);
 }