cmd/compile: fix debugFormat for new export format

Change-Id: Ic3415f3ee643636eab4ff7d2351b8ad0dae62895
Reviewed-on: https://go-review.googlesource.com/22792
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1103,7 +1103,7 @@ func (p *importer) int64() int64 {
 }
 
 func (p *importer) string() string {
-	if debugFormat {
+	if p.debugFormat {
 		p.marker('s')
 	}
 	// if the string was seen before, i is its index (>= 0)