vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

vendor/github.com/gogo/protobuf/test/custom/custom.go

@@ -1,4 +1,6 @@
-// Copyright (c) 2013, Vastech SA (PTY) LTD. All rights reserved.
+// Protocol Buffers for Go with Gadgets
+//
+// Copyright (c) 2013, The GoGo Authors. All rights reserved.
 // http://github.com/gogo/protobuf
 //
 // Redistribution and use in source and binary forms, with or without
@@ -34,7 +36,6 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
-	"unsafe"
 )
 
 type Uint128 [2]uint64
@@ -51,7 +52,15 @@ func (u Uint128) MarshalTo(data []byte) (n int, err error) {
 }
 
 func GetLittleEndianUint64(b []byte, offset int) uint64 {
-	return *(*uint64)(unsafe.Pointer(&b[offset]))
+	v := uint64(b[offset+7]) << 56
+	v += uint64(b[offset+6]) << 48
+	v += uint64(b[offset+5]) << 40
+	v += uint64(b[offset+4]) << 32
+	v += uint64(b[offset+3]) << 24
+	v += uint64(b[offset+2]) << 16
+	v += uint64(b[offset+1]) << 8
+	v += uint64(b[offset])
+	return v
 }
 
 func PutLittleEndianUint64(b []byte, offset int, v uint64) {