gas: avoid bignum related errors when processing .linefile

gas: avoid bignum related errors when processing .linefile

Any construct which to the scrubber looks like a C preprocessor
line/file "directive" is converted to .linefile, but the amount of
checking the scrubber does is minimal (albeit it does let through only
decimal digits for the line part of the contruct). Since the scrubber
conversion is further tied to # being a line comment character, anything
which upon closer inspection turns out not to be a line/file "directive"
is supposed to be treated as a comment, i.e. ignored. Therefore we
cannot use get_absolute_expression(), as this may raise errors. Open-
code the function instead, treating everything not resulting in
O_constant as a comment as well.

Furthermore also bounds-check the parsed value. This bounds check tries
to avoid implementation defined behavior (which may be the raising of an
implementation defined signal), but for now makes the assumption that
int has less than 64 bits. The way bfd_signed_vma (which is what offsetT
aliases) is defined in bfd.h for the BFD64 case I cannot really see a
clean way of avoiding this assumption. Omitting the #ifdef, otoh, would
risk "condition is always false" warnings by compilers.

Convert get_linefile_number() to return bool at this occasion as well.