gas: maintain O_constant signedness in more cases

Unary '~' doesn't really produce an unsigned result. Neither does
subtraction (unless taking operand values into consideration). And an
abstract operator applied to two operands which aren't both unsigned
can't be assumed to yield an unsigned result; exceptions are
- shifts, where only signedness of the left hand operand matters,
- comparisons, which - unlike unary '!' - produce signed results (they
  deliver 0 or ~0, as opposed to '!', which yields 0 or 1),
- logical operators (yielding 0 or 1 and hence treated like unary '!').

While doing this (specifically while extending the all/quad testcase),
update .quad and .8byte documentation: With 64-bit architectures now
being common, it is highly inappropriate to state that these directives
unconditionally require bignums.