[dev.regabi] test: run abi/regabipragma test with -c=1

Currently, we call Warnl in SSA backend when we see a function
(defined or called) with regparams pragma. Calling Warnl in
concurrent environment is racy. As the debugging output is
temporary, for testing purposes we just pass -c=1. We'll remove
the pragma and the debugging print some time soon.

Change-Id: I6f925a665b953259453fc458490c5ff91f67c91a
Reviewed-on: https://go-review.googlesource.com/c/go/+/291710
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>

test/abi/regabipragma.go

@@ -1,4 +1,4 @@
-// runindir
+// runindir -gcflags=-c=1
 // +build !windows
 
 // Copyright 2021 The Go Authors. All rights reserved.

test/run.go

@@ -902,6 +902,7 @@ func (t *test) run() {
 		if *linkshared {
 			cmd = append(cmd, "-linkshared")
 		}
+		cmd = append(cmd, flags...)
 		cmd = append(cmd, ".")
 		out, err := runcmd(cmd...)
 		if err != nil {