Parse out arm isa targets from dumpmachine

Parse out arm isa targets from dumpmachine

The current parsing logic of the dumpmachine tuple lacks any arm
cases which means tgt_isa never gets set, so for all arm targets,
we get detected as generic-gnu.  Add some basic arm checks here
so the automatic detection logic works.

Change-Id: Ie5e98142876025c6708604236bc519c0bdb09319