gas/arc: Limit special handling of t/nt flag to ARCv2

gas/arc: Limit special handling of t/nt flag to ARCv2

In a later commit I'll be adding a new version of the ".nt" flag for an
ARC700 extension (NPS400) which does not require this same special
handling.

In this commit I have restricted the special flag handling to only apply
if we are assembling for ARCv2.  This is a restructuring commit, and
there should be no user visible changes after this commit.

gas/ChangeLog:

	* config/tc-arc.c (assemble_insn): Only handle ".t" and ".nt"
	specially for ARCv2.