Revert "Revert "Lose the hand-written futex assembler.""

The problem with the original patch was that using syscall(3) means that
errno can be set, but pthread_create(3) was abusing the TLS errno slot as
a pthread_mutex_t for the thread startup handshake.

There was also a mistake in the check for syscall failures --- it should
have checked against -1 instead of 0 (not just because that's the default
idiom, but also here because futex(2) can legitimately return values > 0).

This patch stops abusing the TLS errno slot and adds a pthread_mutex_t to
pthread_internal_t instead. (Note that for LP64 sizeof(pthread_mutex_t) >
sizeof(uintptr_t), so we could potentially clobber other TLS slots too.)

I've also rewritten the LP32 compatibility stubs to directly reuse the
code from the .h file.

This reverts commit 75c55ff84e.

Bug: 15195455
Change-Id: I6ffb13e5cf6a35d8f59f692d94192aae9ab4593d

libc/private/bionic_tls.h

@@ -46,32 +46,27 @@ __BEGIN_DECLS
  ** pre-allocated slot directly for performance reason).
  **/
 
-/* Well-known TLS slots. What data goes in which slot is arbitrary unless otherwise noted. */
+// Well-known TLS slots. What data goes in which slot is arbitrary unless otherwise noted.
 enum {
-  TLS_SLOT_SELF = 0, /* The kernel requires this specific slot for x86. */
+  TLS_SLOT_SELF = 0, // The kernel requires this specific slot for x86.
   TLS_SLOT_THREAD_ID,
   TLS_SLOT_ERRNO,
 
-  /* This slot in the child's TLS is used to synchronize the parent and child
-   * during thread initialization. The child finishes with this mutex before
-   * running any code that can set errno, so we can reuse the errno slot. */
-  TLS_SLOT_START_MUTEX = TLS_SLOT_ERRNO,
-
-  /* These two aren't used by bionic itself, but allow the graphics code to
-   * access TLS directly rather than using the pthread API. */
+  // These two aren't used by bionic itself, but allow the graphics code to
+  // access TLS directly rather than using the pthread API.
   TLS_SLOT_OPENGL_API = 3,
   TLS_SLOT_OPENGL = 4,
 
-  /* This slot is only used to pass information from the dynamic linker to
-   * libc.so when the C library is loaded in to memory. The C runtime init
-   * function will then clear it. Since its use is extremely temporary,
-   * we reuse an existing location that isn't needed during libc startup. */
+  // This slot is only used to pass information from the dynamic linker to
+  // libc.so when the C library is loaded in to memory. The C runtime init
+  // function will then clear it. Since its use is extremely temporary,
+  // we reuse an existing location that isn't needed during libc startup.
   TLS_SLOT_BIONIC_PREINIT = TLS_SLOT_OPENGL_API,
 
-  TLS_SLOT_STACK_GUARD = 5, /* GCC requires this specific slot for x86. */
+  TLS_SLOT_STACK_GUARD = 5, // GCC requires this specific slot for x86.
   TLS_SLOT_DLERROR,
 
-  TLS_SLOT_FIRST_USER_SLOT /* Must come last! */
+  TLS_SLOT_FIRST_USER_SLOT // Must come last!
 };
 
 /*