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>

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -414,8 +414,9 @@ var genericBlocks = []blockData{
 
 func init() {
 	archs = append(archs, arch{
-		name:   "generic",
-		ops:    genericOps,
-		blocks: genericBlocks,
+		name:    "generic",
+		ops:     genericOps,
+		blocks:  genericBlocks,
+		generic: true,
 	})
 }