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>