arc: Add 'set disassembler-options' support

Implement ARC target support for passing options to the disassembler
through the command interface. e.g.:

gdb> set disassembler-options cpu=hs38_linux ...

gdb/ChangeLog:

	* NEWS: Document 'set disassembler-options' support for the ARC
	target.
	* arc-tdep.c (arc_gdbarch_init): Set
	'gdbarch_valid_disassembler_options'.

gdb/doc/ChangeLog:

	* gdb.texinfo (Source and Machine Code): Document 'set
	disassembler-options' support for the ARC target.

gdb/testsuite/ChangeLog:

	* gdb.arch/arc-disassembler-options.exp: New test.
	* gdb.arch/arc-disassembler-options.s: New test source.