gas: multi-byte warning adjustments

gas: multi-byte warning adjustments

First input_scrub_next_buffer()'s invocation was wrong, leading to input
only being checked from the last newline till the end of the current
buffer. Correcting the invocation, however, leads to duplicate checking
unless -f (or the #NO_APP equivalent thereof) is in effect. Move the
invocation to input_file_give_next_buffer(), to restrict it accordingly.

Then, when macros contain multi-byte characters, warning about them
again in every expansion isn't useful. Suppress such warnings from
sb_scrub_and_add_sb().