gas: correctly deal with line comments when not preprocessing

gas: correctly deal with line comments when not preprocessing

Internal naming of functions / data as well as commentary mixes lines
and statements. It is presumably this confusion which has led to the
wrong use of ignore_rest_of_line() when dealing with line comments in
read_a_source_file(). We shall not (silently) produce different output
depending on whether -f is passed (for suitable input).

Introduce two new helper macros, intended to be used in favor of open-
coded accesses to is_end_of_line[]. To emphasize the difference, convert
ignore_rest_of_line() right away, including adjustments to its comments.

Since most targets have # in line_comment_chars[], add a target-
independent test for that, plus an x86-only one also checking for non-#
to work as intended.