[ARC] Misc minor edits/fixes

[ARC] Misc minor edits/fixes

The code supporting -mspfp, -mdpfp, and -mfpuda options are in
sections of code that are commented as being for backward
compatibility only, and having no effect. However, they do have an
effect, enabling the SPX, DPX, and DPA instruction subclasses
respectively. This commit moves the code supporting these options
away from the comments indicating that they are dummy options, and
also fixes a small issue where -mnps400 had the additional effect
of enabling SPX instructions.

A couple of other minor edits (that make no functional change) are
also included.

gas/ChangeLog:

        * config/tc-arc.c (options, md_longopts, md_parse_option):
        Move -mspfp, -mdpfp and -mfpuda out of the sections for
        dummy options. Correct erroneous enabling of SPFP
        instructions when using -mnps400.

include/ChangeLog:

        * opcode/arc.h: Make insn_class_t alphabetical again.

opcodes/ChangeLog:

        * arc-opc.c: Correct description of availability of NPS400
        features.