gas: avoid bogus warnings in false branches of conditional

gas: avoid bogus warnings in false branches of conditional

The construct being added to the cond.s test case otherwise triggered
both the "missing closing ..." and the "stray ..." (twice) warnings in
_find_end_of_line(). As that code fragments suggests, this is needed to
support (include) files that can be used for both assembler .include
and compiler #include directives.