cmd/vet: add test case for ppc64{,le}

Adapted from the mips64 test case.

Fixes #17745.

Change-Id: I46f0900028adb936dcab2cdc701ea11d0a3cb95e
Reviewed-on: https://go-review.googlesource.com/32611
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>