runtime: correct runtime structfield type to match reflect

The offset field in structfield has changed to offsetAnon, and now
requires a shift to get the actual offset value.

Fixes golang/go#23391

Change-Id: I663997eb4a6e73c6e8333e29c305df469db94954
Reviewed-on: https://go-review.googlesource.com/92275
Reviewed-by: Than McIntosh <[email protected]>

Author: ianlancetaylor

libgo/go/runtime/cgocall.go

@@ -189,7 +189,7 @@ func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
 			return
 		}
 		for _, f := range st.fields {
-			cgoCheckArg(f.typ, add(p, f.offset), true, top, msg)
+			cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
 		}
 	case kindPtr, kindUnsafePointer:
 		if indir {

libgo/go/runtime/type.go

@@ -113,11 +113,19 @@ type ptrtype struct {
 }
 
 type structfield struct {
-	name    *string // nil for embedded fields
-	pkgPath *string // nil for exported Names; otherwise import path
-	typ     *_type  // type of field
-	tag     *string // nil if no tag
-	offset  uintptr // byte offset of field within struct
+	name       *string // nil for embedded fields
+	pkgPath    *string // nil for exported Names; otherwise import path
+	typ        *_type  // type of field
+	tag        *string // nil if no tag
+	offsetAnon uintptr // byte offset of field<<1 | isAnonymous
+}
+
+func (f *structfield) offset() uintptr {
+	return f.offsetAnon >> 1
+}
+
+func (f *structfield) anon() bool {
+	return f.offsetAnon&1 != 0
 }
 
 type structtype struct {