gas: consistently drop trailing whitespace when scrubbing

From especially the checks for the two separator forms it appears to
follow that the construct being touched is about trailing whitespace. In
such a case, considering that for many targets ordinary and line comment
chars overlap, take into account that line comment chars override
ordinary ones in lex[] (logic elsewhere in do_scrub_chars() actually
depends on that ordering, and also accounts for this overriding).

Plus of course IS_NEWLINE() would better also be consulted. Note also
that the DOUBLESLASH_LINE_COMMENTS change should generally have no
effect just yet; it's a prereq for a later change but better fits here.

Leave respective comments as well, and update documentation to correct
which comment form is actually replaced by a single blank (i.e. neither
the ones starting with what {,tc_}comment_chars[] has nor the ones
starting with what line_comment_chars[] has).