x86_64: Undef SHADOW_STACK_POINTER_OFFSET last

Since SHADOW_STACK_POINTER_OFFSET is defined in jmp_buf-ssp.h, we must
undef SHADOW_STACK_POINTER_OFFSET after including <jmp_buf-ssp.h>.

	* sysdeps/unix/sysv/linux/x86_64/____longjmp_chk.S: Undef
	SHADOW_STACK_POINTER_OFFSET after including <jmp_buf-ssp.h>.