dashboard: add OpenBSD 6.4 to BuildConfig.buildSubrepos

By now, OpenBSD 6.4 has proven reliable, so we want it to be used
for building subrepos as well.

Updates golang/go#26833

Change-Id: I026207865514a026f86936140a16dd707f9594ea
Reviewed-on: https://go-review.googlesource.com/c/148888
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

dashboard/builders.go

@@ -814,6 +814,7 @@ func (c *BuildConfig) buildSubrepos() bool {
 		"linux-s390x-ibm",
 		"openbsd-386-60", "openbsd-amd64-60",
 		"openbsd-386-62", "openbsd-amd64-62",
+		"openbsd-386-64", "openbsd-amd64-64",
 		"netbsd-amd64-8_0",
 		"netbsd-386-8_0",
 		"plan9-386",