dashboard: add known issue for plan9-arm
dashboard: add known issue for plan9-arm

For golang/go#52653.
Updates golang/go#49338.

Change-Id: Ib23be56bd79bce8b3168cb40064916a2a91dbb88
Reviewed-on: https://go-review.googlesource.com/c/build/+/408697
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build