[dev.cc] cm/asm: fix up arm after cross-check with 5a
As with the previous round for ppc64, this CL fixes a couple of things
that 5a supported but asm did not, both simple.
1) Allow condition code on MRC instruction; this was marked as a TODO.
2) Allow R(n) notation in ARM register shifts. The code needs a rethink
but the tests we're leading toward will make the rewrite easier to test and
trust.