completions/gcc: Cleanup options

- Remove duplicated options - we had `-type` 9 times!
- Remove deprecated options and synonyms
- Make descriptions shorter, even removing some - when they're inscrutable they might as well not be there.

Really, 99.8% of these options are of interest to nobody except possibly (a subset of) gcc developers, so it pays to have *less* on your screen that you don't use anyway.