dashboard: switch OpenBSD trybot from 6.2 to 6.4

OpenBSD 6.4 is the current OpenBSD release, while 6.2 has just stopped
being supported. The new 6.4 builder has proven to be stable and
reliable, so start using it for trybots as well.

Looking at the non-trybot build times for the Go repository with
OpenBSD 6.2 and 6.4 builders to get a sense of the build speed,
it seems OpenBSD 6.4 is about 15% slower (1200~ seconds on 6.2 vs
1380~ seconds on 6.4). This might be due to security-oriented changes
in OpenBSD 6.3 and 6.4 that were done to mitigate Spectre/Meltdown
vulnerabilities.

CL 145022 has helped recover some lost performance by re-enabling SMT.

OpenBSD trybots seem to take around 6 minutes now, so there's
some room to make them slower before they're the bottleneck.
Try to update for now, and if it becomes a bottleneck, then
we can prioritize investigating ways to improve OpenBSD 6.4
builder performance further if possible.

References:

• https://www.openbsd.org/64.html
• https://www.openbsd.org/63.html
• https://man.openbsd.org/sysctl.2#HW_SMT_2

Fixes golang/go#26833

Change-Id: I42c136fab800df46866a95017a8f70047b37f847
Reviewed-on: https://go-review.googlesource.com/c/144777
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: dmitshur

dashboard/builders.go

@@ -1371,7 +1371,7 @@ func init() {
 		Name:              "openbsd-amd64-62",
 		HostType:          "host-openbsd-amd64-62",
 		ShouldRunDistTest: noTestDir,
-		tryBot:            defaultTrySet(),
+		tryBot:            nil,
 		numTestHelpers:    0,
 		numTryTestHelpers: 5,
 		MaxAtOnce:         1,
@@ -1380,7 +1380,7 @@ func init() {
 		Name:              "openbsd-amd64-64",
 		HostType:          "host-openbsd-amd64-64",
 		ShouldRunDistTest: noTestDir,
-		tryBot:            nil,
+		tryBot:            defaultTrySet(),
 		numTestHelpers:    0,
 		numTryTestHelpers: 5,
 		MaxAtOnce:         1,