x86: make I disassembler macro available for new use

The {Q|Q} construct on lgdt/lidt and the slightly different {Q|IQ}
struck me as odd when encountering. Realizing they both have the same
effect, let's free up the I macro by using the former form of construct
where needed (there aren't that many uses overall).

With there now being several multi-character macros also re-do "alt"
handling slightly: Terminate it when finding the closing brace, rather
than after the next single character. Also set the flag only when
actually in Intel syntax mode.