cmd/internal/gc: make class uint8 throughout

This prevents conversion noise.

No functional changes. Passes toolstash -cmp.

Change-Id: I238adf28680f875217827931a689ce7f19a9b371
Reviewed-on: https://go-review.googlesource.com/8164
Reviewed-by: David Chase <drchase@google.com>
diff --git a/src/cmd/internal/gc/export.go b/src/cmd/internal/gc/export.go
index f950889..76bb55a 100644
--- a/src/cmd/internal/gc/export.go
+++ b/src/cmd/internal/gc/export.go
@@ -57,7 +57,7 @@
 	return sym.Pkg == localpkg && exportname(sym.Name)
 }
 
-func autoexport(n *Node, ctxt int) {
+func autoexport(n *Node, ctxt uint8) {
 	if n == nil || n.Sym == nil {
 		return
 	}