perf: add linux/arm64 dropdown option to dashboard

Everything is already wired up and I can manually select data for
linux/arm64 by modifying the query params, so all that's left is adding
a dropdown option to make it easier to find.

Fixes golang/go#70231.

Change-Id: Ide7427f45a042e1ceca75763b440347daf5dabad
Reviewed-on: https://go-review.googlesource.com/c/build/+/627275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

perf/app/dashboard/index.html

@@ -61,6 +61,7 @@ license that can be found in the LICENSE file.
 				Platform:
 				<select id="platform-select" name="platform">
 					<option>linux/amd64</option>
+					<option>linux/arm64</option>
 				</select>
 				Duration (days):
 				<div class="Dashboard-duration">