Add multibyte character warning option to the assembler.

Add multibyte character warning option to the assembler.

	* as.c (parse_args): Add support for --multibyte-handling.
	* as.h (multibyte_handling): Declare.
	* app.c (scan_for_multibyte_characters): New function.
	(do_scrub_chars): Call the new function if multibyte warning is
	enabled.
	* input-scrub,c (input_scrub_next_buffer): Call the multibyte
	scanning function if multibyte warnings are enabled.
	* symbols.c (struct symbol_flags): Add multibyte_warned bit.
	(symbol_init): Call the multibyte scanning function if multibyte
	symbol warnings are enabled.
	(S_SET_SEGMENT): Likewise.
	* NEWS: Mention the new feature.
	* doc/as.texi: Document the new feature.
	* testsuite/gas/all/multibyte.s: New test source file.
	* testsuite/gas/all/multibyte1.d: New test driver file.
	* testsuite/gas/all/multibyte1.l: New test expected output.
	* testsuite/gas/all/multibyte2.d: New test driver file.
	* testsuite/gas/all/multibyte2.l: New test expected output.
	* testsuite/gas/all/gas.exp: Run the new tests.