hash/crc32: invert build tags for go implementation

hash/crc32: invert build tags for go implementation

It seems cleaner and more consistent with other files to list the
architectures that have assembly implementations rather than to
list those that do not.

This means we don't have to add s390x and future platforms to this
list.

Change-Id: I2ad3f66b76eb1711333c910236ca7f5151b698e5
Reviewed-on: https://go-review.googlesource.com/21770
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>