x86/AT&T: restrict recognition of the "absolute branch" prefix character

x86/AT&T: restrict recognition of the "absolute branch" prefix character

While in principle merely rejecting this for .insn would be sufficient
for the purposes there, be more generic and reject it for anything that
isn't going to be a branch: All elements of same-mnemonic template
groups either are branches, or are not, and the few cases possibly
requiring a 2nd parsing pass aren't affected either. This then also
improves diagnostics for misuses like

	inc	*%eax
	incl	%fs:*(%eax)
	add	*$1, %eax