cmd/cgo: for C bitfields use only valid Go integer types

Fixes #22958

Change-Id: Ib078a5f6e1105a2afca77c6d9a05f65ddf5d9010
Reviewed-on: https://go-review.googlesource.com/81435
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

misc/cgo/test/issue22958.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+// Test handling of bitfields.
+
+/*
+typedef struct {
+	unsigned long long f8  : 8;
+	unsigned long long f16 : 16;
+	unsigned long long f24 : 24;
+	unsigned long long f32 : 32;
+	unsigned long long f40 : 40;
+	unsigned long long f48 : 48;
+	unsigned long long f56 : 56;
+	unsigned long long f64 : 64;
+} issue22958Type;
+*/
+import "C"
+
+// Nothing to run, just make sure this compiles.
+var Vissue22958 C.issue22958Type

src/cmd/cgo/gcc.go

@@ -2388,7 +2388,9 @@ func (c *typeConv) Struct(dt *dwarf.StructType, pos token.Pos) (expr *ast.Struct
 		size := t.Size
 		talign := t.Align
 		if f.BitSize > 0 {
-			if f.BitSize%8 != 0 {
+			switch f.BitSize {
+			case 8, 16, 32, 64:
+			default:
 				continue
 			}
 			size = f.BitSize / 8