cmd/dist: skip the staleness check for 'cmd' when testing on aix-ppc64

cmd/dist: skip the staleness check for 'cmd' when testing on aix-ppc64

The gcc toolchain on the aix-ppc64 builder apparently does not achieve
reproducible builds for packages that use cgo, which causes the
binaries in cmd that use package "net" (cmd/go, cmd/pprof, and
cmd/trace) to appear stale whenever the Go build cache is cleared.

For now, we work around the staleness by rebuilding std and simply not
checking whether cmd is stale.

For #56896.
Updates #47257.

Change-Id: I15f86e72dee53904b881710d5d5d613872361510
Reviewed-on: https://go-review.googlesource.com/c/go/+/452680
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ayappan Perumal <ayappanec@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>