[ARC] Fix handling of cpu=... disassembler option value

[ARC] Fix handling of cpu=... disassembler option value

There is a bug in handling of cpu=... disassembler option in case there are
other options after it, for example, `cpu=EM,dsp'.  In this case `EM,dsp' is
treated as an option value, and strcasecmp reports is as non-equal to "EM".
This is fixed by using disassembler_options_cmp function, which compares string
treating `,' the same way as `\0'.

This function also solves a problem with option order in parse_option.
Previously, if several option had same prefix (e.g. fpud, fpuda), then the
longer one should have been compared first, otherwise when longer option is
passed it would be treated as a short one, because

  CONST_STRNEQ ("fpud", "fpuda")

would be true.  The order of options was correct for ARC, so there were no
bugs per se, but with disassembler_option_cmp there is no risk of such a bug
being introduced in the future.

opcodes/ChangeLog:

yyyy-mm-dd  Anton Kolesov  <Anton.Kolesov@synopsys.com>

	* arc-dis.c (parse_option): Use disassembler_options_cmp to compare
	disassembler option strings.
	(parse_cpu_option): Likewise.

binutils/ChangeLog

yyyy-mm-dd  Anton Kolesov  <Anton.Kolesov@synopsys.com>

	* testsuite/binutils-all/arc/double_store.s: New file.
	* testsuite/binutils-all/arc/objdump.exp: Tests for disassembler
	options.
	(do_objfile): New function.
	(check_assembly): Likewise.