x86/gas: support quoted address scale factor in AT&T syntax

An earlier attempt (e68c3d59 ["x86: better respect quotes in
parse_operands()"]) needed undoing (cc0f9635 ["x86: permit
parenthesized expressions again as addressing scale factor"]) as far its
effect here went. As indicated back then, the issue is the backwards
scanning of the operand string to find the matching opening parenthesis.
Switch to forward scanning, finding the last outermost unquoted opening
parenthesis (which is the one matching the trailing closing one).