PR22714, Assembler preprocessor loses track of \@

PR22714, Assembler preprocessor loses track of \@

The PR22714 testcase is such that the input buffer processed by
do_scrub_chars ends on this line

1: bug "Returning to usermode but unexpected PSR bits set?", \@

right at the backslash.  (The line is part of a macro definition.)
The next input buffer then starts with '@' which starts a comment on
ARM, and the check for \@ fails due to to == tostart.  Now it would be
possible to simply access to[-1] in this particular case, but that's
ugly, and to be absolutely safe from people deliberately trying to
crash gas we'd need the read.c:read_a_source_file buffer passed to
do_scrub_chars to have a single byte pad at the start.

	PR 22714
	* app.c (last_char): New static var.
	(struct app_save): Add last_char field.
	(app_push, app_pop): Handle it.
	(do_scrub_chars): Use last_char in test for "\@".  Set last_char.