[dev.regabi] cmd/compile: fix -h

The compile -h flag is *meant* to panic, so you can see the stack
trace where the error is being printed. Make it do that again.

Change-Id: Ieb0042863582d7a4c5d08d2f866a144962915b06
Reviewed-on: https://go-review.googlesource.com/c/go/+/274116
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/main.go

@@ -43,6 +43,9 @@ func hidePanic() {
 		// about a panic too; let the user clean up
 		// the code and try again.
 		if err := recover(); err != nil {
+			if err == "-h" {
+				panic(err)
+			}
 			base.ErrorExit()
 		}
 	}