cmd/compile: use correct export function (fix debugFormat)

Tested with debugFormat enabled and running
(export GO_GCFLAGS=-newexport; sh all.bash).

Change-Id: If7d43e1e594ea43c644232b89e670f7abb6b003e
Reviewed-on: https://go-review.googlesource.com/22033Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -350,7 +350,7 @@ func export(out *bufio.Writer, trace bool) int {
 	if p.trace {
 		p.tracef("\n")
 	}
-	p.tag(-1) // invalid index terminates list
+	p.int(-1) // invalid index terminates list
 
 	// for self-verification only (redundant)
 	p.int(objcount)