Improve handling of REPT pseudo op with a negative count.

Improve handling of REPT pseudo op with a negative count.

	PR 22324
	* read.c (s_rept): Use size_t type for count parameter.
	(do_repeat): Change type of count parameter to size_t.
	Issue an error is the count parameter is negative.
	(do_repeat_with_expression): Likewise.
	* read.h: Update prototypes for do_repeat and
	do_repeat_with_expression.
	* doc/as.texinfo (Rept): Document that a zero count is allowed but
	negative counts are not.
	* config/tc-rx.c (rx_rept): Use size_t type for count parameter.
	* config/tc-tic54x.c (tic54x_loop): Cast count parameter to size_t
	type.
	* testsuite/gas/macros/end.s: Add a test using a negative repeat
	count.
	* testsuite/gas/macros/end.l: Add expected error message.