gas/Dwarf: improve debug info generation from .irp and alike blocks

gas/Dwarf: improve debug info generation from .irp and alike blocks

Tying the bumping of the logical line number to reading from the
original source file looks wrong: Upon finishing of the processing of an
sb the original values will be restored anyway. Yet without bumping the
line counter uses of .line inside e.g. an .irp construct won't have the
intended effect: Such uses may be necessary to ensure proper debug info
is emitted in particular when switching sections inside the .irp body,
as dwarf2_gen_line_info() would bail without doing anything when it
finds the line number unchanged from what it saw last.