x86: Add feature_1 to tcbhead_t [BZ #22563]

On x86, padding in struct __jmp_buf_tag is used for shadow stack pointer
to support Shadow Stack in Intel Control-flow Enforcemen Technology.
cancel_jmp_buf has been updated to include saved_mask so that it is as
large as struct __jmp_buf_tag.  We must suport the old cancel_jmp_buf
in existing binaries.  Since symbol versioning doesn't work on
cancel_jmp_buf, feature_1 is added to tcbhead_t so that setjmp and
longjmp can check if shadow stack is enabled.  NB: Shadow stack is
enabled only if all modules are shadow stack enabled.

	[BZ #22563]
	* sysdeps/i386/nptl/tcb-offsets.sym (FEATURE_1_OFFSET): New.
	* sysdeps/i386/nptl/tls.h (tcbhead_t): Add feature_1.
	* sysdeps/x86_64/nptl/tcb-offsets.sym (FEATURE_1_OFFSET): New.
	* sysdeps/x86_64/nptl/tls.h (tcbhead_t): Rename __glibc_unused1
	to feature_1.