internal/gclayout: make gclayout values constants

The previous versions calculated at init() prevented `interp` from running
in many cases, increasing compile times due to the increased need to revert
the partially interpreted results and also increasing binary runtime because
fewer optimizations had happened during interp.

src/internal/reflectlite/type.go

@@ -708,16 +708,16 @@ func (r *RawType) gcLayout() unsafe.Pointer {
 	kind := r.Kind()
 
 	if kind < String {
-		return gclayout.NoPtrs
+		return gclayout.NoPtrs.AsPtr()
 	}
 
 	switch kind {
 	case Pointer, UnsafePointer, Chan, Map:
-		return gclayout.Pointer
+		return gclayout.Pointer.AsPtr()
 	case String:
-		return gclayout.String
+		return gclayout.String.AsPtr()
 	case Slice:
-		return gclayout.Slice
+		return gclayout.Slice.AsPtr()
 	}
 
 	// Unknown (for now); let the conservative pointer scanning handle it