[PATCH]: finish .cpu support for ARM

[Tim Northover]

> Sure... with a fixed test case now.

Sorry to do this (I should have looked harder the first time), but it
still seems a little lax. The ".cpu cortex-a8" could fail and we'd
never notice. I'd probably suggest separating stdout and stderr (%t
and %t2 are temporary files that llvm-lit understands).

You can check that stdout does contain the valid .cpu directive and
the dsb instruction (only once!), and that stderr has the errors
you're already looking for.

I'd suggest separate files because mixing them with "2>&1" tends to be
OK if you're only checking one, but can lead to race conditions if
you're interested in both.

Cheers.

Tim.