Default to enabling colored disassembly if output is to a terminal.

Default to enabling colored disassembly if output is to a terminal.

	PR 29457
	* objdump.c (disassembler_color): Change type to an enum.
	(disassembler_extended_color): Remove.
	(usage): Update.
	(objdump_color_for_assembler_style): Update.
	(main): Update initialisation of disassembler_color.  If not
	initialised via a command line option, set based upon terminal
	output.
	* doc/binutils.texi: Update description of disassmbler-color
	option.
	* testsuite/binutils-all/arc/objdump.exp: Add
	--disassembler-color=off option when disassembling.
	* testsuite/binutils-all/arm/objdump.exp: Likewise.