i386 TLS_INIT_TP might produce bogus asm changing stack pointer [BZ #17319]

TLS_INIT_TP in sysdeps/i386/nptl/tls.h uses some hand written asm to
generate a set_thread_area that might result in exchanging ebx and esp
around the syscall causing introspection tools like valgrind to loose
track of the user stack. Just use INTERNAL_SYSCALL which makes sure
esp isn't changed arbitrarily.

Before the patch the code would generate:

mov    $0xf3,%eax
movl   $0xfffff,0x8(%esp)
movl   $0x51,0xc(%esp)
xchg   %esp,%ebx
int    $0x80
xchg   %esp,%ebx

Using INTERNAL_SYSCALL instead will generate:

movl   $0xfffff,0x8(%esp)
movl   $0x51,0xc(%esp)
xchg   %ecx,%ebx
mov    $0xf3,%eax
int    $0x80
xchg   %ecx,%ebx

Thanks to Florian Weimer for analysing why the original code generated
the bogus esp usage:

  _segdescr.desc happens to be at the top of the stack, so its address
  is in %esp.  The asm statement says that %3 is an input, so its value
  will not change, and GCC can use %esp as the input register for the
  expression &_segdescr.desc.  But the constraints do not fully describe
  the asm statement because the %3 register is actually modified, albeit
  only temporarily.

	[BZ #17319]
	* sysdeps/i386/nptl/tls.h (TLS_INIT_TP): Use INTERNAL_SYSCALL
	to call set_thread_area instead of hand written asm.
	(__NR_set_thread_area): Removed define.
	(TLS_FLAG_WRITABLE): Likewise.
	(__ASSUME_SET_THREAD_AREA): Remove check.
	(TLS_EBX_ARG): Remove define.
	(TLS_LOAD_EBX): Likewise.
Fix hang on fork

If e.g. a signal is being received while we are running fork(), the signal
thread may be having our SS lock when we make the space copy, and thus in the
child we can not take the SS lock any more.

* sysdeps/mach/hurd/fork.c (__fork): Lock SS->lock around __proc_dostop call.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>