x86: Use pad in pthread_unwind_buf to preserve shadow stack register

x86: Use pad in pthread_unwind_buf to preserve shadow stack register

The pad array in struct pthread_unwind_buf is used by setjmp to save
shadow stack register.  We assert that size of struct pthread_unwind_buf
is no less than offset of shadow stack pointer + shadow stack pointer
size.

Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
these with thread cancellation, call setjmp, but never return after
__libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
__libc_longjmp on x86, doesn't need to restore shadow stack register.
__libc_longjmp, which is a private interface for thread cancellation
implementation in libpthread, is changed to call __longjmp_cancel,
instead of __longjmp.  __longjmp_cancel is a new internal function
in libc, which is similar to __longjmp, but doesn't restore shadow
stack register.

The compatibility longjmp and siglongjmp in libpthread.so are changed
to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
restore shadow stack register.

Tested with build-many-glibcs.py.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
	* nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
	handlers after setjmp.
	* setjmp/longjmp.c (__libc_longjmp): Don't define alias if
	defined.
	* sysdeps/unix/sysv/linux/x86/setjmpP.h: Include
	<libc-pointer-arith.h>.
	(_JUMP_BUF_SIGSET_BITS_PER_WORD): New.
	(_JUMP_BUF_SIGSET_NSIG): Changed to 96.
	(_JUMP_BUF_SIGSET_NWORDS): Changed to use ALIGN_UP and
	_JUMP_BUF_SIGSET_BITS_PER_WORD.
	* sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
	* sysdeps/x86/__longjmp_cancel.S: New file.
	* sysdeps/x86/longjmp.c: Likewise.
	* sysdeps/x86/nptl/pt-longjmp.c: Likewise.