Tidy gas/expr.c static state

	* expr.c (seen, nr_seen): Make file scope.
	(expr_begin): Clear seen, nr_seen, and expr_symbol_lines.
	(expr_end): New function.
	* expr.h (expr_end): Declare.
	* output-file.c (output_file_close): Call expr_end.
	* config/tc-hppa.c (expr_end): Rename to expr_parse_end.
	* config/tc-mips.c: Likewise.
	* config/tc-riscv.c: Likewise.
	* config/tc-sparc.c: Likewise.