encoding/gob: increase "tooBig" from 1GB to 8GB on 64-bit machines

A little shift magic makes it easy to adjust the maximum buffer
size on machines with larger integers.

Fixes #27635

Change-Id: I1f26b07a363fbb9730df2377052475fa88bbb781
Reviewed-on: https://go-review.googlesource.com/c/143678
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/gob/decoder.go

@@ -12,10 +12,10 @@ import (
 	"sync"
 )
 
-// tooBig provides a sanity check for sizes; used in several places.
-// Upper limit of 1GB, allowing room to grow a little without overflow.
-// TODO: make this adjustable?
-const tooBig = 1 << 30
+// tooBig provides a sanity check for sizes; used in several places. Upper limit
+// of is 1GB on 32-bit systems, 8GB on 64-bit, allowing room to grow a little
+// without overflow.
+const tooBig = (1 << 30) << (^uint(0) >> 62)
 
 // A Decoder manages the receipt of type and data information read from the
 // remote side of a connection.