cmd/compile: start on ARM port

cmd/compile: start on ARM port

Start working on arm port.  Gets close to correct
code for fibonacci:
    func fib(n int) int {
        if n < 2 {
            return n
        }
        return fib(n-1) + fib(n-2)
    }

Still a lot to do, but this is a good starting point.

Cleaned up some arch-specific dependencies in regalloc.

Change-Id: I4301c6c31a8402168e50dcfee8bcf7aee73ea9d5
Reviewed-on: https://go-review.googlesource.com/21000
Reviewed-by: David Chase <drchase@google.com>