[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>