dashboard: add linux-amd64-regabi builder

This adds a builder for the GOEXPERIMENT=regabi configuration of the
compiler. We'll want this builder until we enable regabi by default.

Updates golang/go#40724.

Change-Id: Ib1c7ae1e914116dd8a4440db8ee46d6af3ed1ad6
Reviewed-on: https://go-review.googlesource.com/c/build/+/279673
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index 24036ee..56bc13c 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -599,6 +599,9 @@
 		{b("linux-amd64-staticlockranking@go1.15", "go"), onlyPost},
 		{b("linux-amd64-staticlockranking@go1.14", "go"), none},
 		{b("linux-amd64-staticlockranking", "net"), none},
+
+		{b("linux-amd64-regabi", "go"), onlyPost},
+		{b("linux-amd64-regabi", "net"), none},
 	}
 	for _, tt := range tests {
 		t.Run(tt.br.testName, func(t *testing.T) {