arc/gas: default mach is arc700, initialised in md_begin

arc/gas: default mach is arc700, initialised in md_begin

This commit restructures the selection of the default cpu/mach so that
the choice is made from md_begin (if the user has not provided a command
line choice).  This will reduce the amount of change needed in a later
patch.

At the request of Synopsys, the default architecture changes to ARC700
from this commit, previously the default was a non-existent
super-architecture that contained all instructions from all arc
variants.  There's some clean up associated with removing the default
merged architecture, and a small test fix now that the default is
ARC700.

binutils/ChangeLog:

	* testsuite/binutils-all/objdump.exp (cpus_expected): Add ARC700
	to the architecture list.

gas/ChangeLog:

	* config/tc-arc.c (arc_target): Delay initialisation until
	arc_select_cpu.
	(arc_target_name): Likewise.
	(arc_features): Likewise.
	(arc_mach_type): Likewise.
	(cpu_types): Remove "all" entry.
	(arc_select_cpu): New function, most of the content is from...
	(md_parse_option): ... here.  Call new arc_select_cpu.
	(md_begin): Call arc_select_cpu if needed, default is now arc700.

include/ChangeLog:

	* opcode/arc.h (ARC_OPCODE_BASE): Delete.

opcodes/ChangeLog:

	* arc-opc.c (BASE): Delete.