gas: tighten get_single_number()

Move what the function does closer to what its name says. Add some error
checking, also in its sole caller (implementing entirely undocumented and
- so far - entirely untested behavior).

I really want the -f passed to gas in the new testcase, yet that means the
odd-looking .if expression can't be commented upon
("NUMBERS_WITH_SUFFIX implies in particular no C-style octal numbers").