cmd: update x/arch to pull in CL 315572

Because x/arch is now lazy, this removes the checksum for rsc.io/pdf
from the go.sum file: the requirements of rsc.io/pdf are known not to
be relevant to any package imported within the cmd packages.

For #36460
Updates #36905

Change-Id: I3abb6a8029cd0c9099b592ccb01ca5606c93edec
Reviewed-on: https://go-review.googlesource.com/c/go/+/316110


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>