cmd/vet: go vet on s390x can't handle assembly

[billotosyr]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   tip
2. What operating system and processor architecture are you using (go env)?
   linux s390x
3. What did you do?
   go vet in math/big
4. What did you expect to see?
   not an error
5. What did you see instead?
   : arith_s390x.s: cannot determine architecture for assembly file

I will fix this...

[mundaym]

cc @minux @cherrymui

mip64 is missing from the list in cmd/vet too.

[gopherbot]

CL https://golang.org/cl/22472 mentions this issue.