gas/arc: Use #define from include/opcode/arc.h

gas/arc: Use #define from include/opcode/arc.h

In tc-arc.h we redefine constants that are also defined in the
include/opcode/arc.h header file.  The problem is that changing one
without changing the other resulting in strange crashes.

We could comment both locations to stress the importance of remembering
to update the other location too, or we could just include the
opcode/arc.h header file into the assembler.

Given that other targets include their opcode/ARCH.h header file, and
only having a single definition, where possible, is usually the safest
solution, that's what I switch too with this commit.

gas/ChangeLog:

	* config/tc-arc.h: Include 'opcode/arc.h'.
	(MAX_INSN_ARGS): Delete.
	(MAX_INSN_FLGS): Delete.