internal/syscall/windows: increase registry.ExpandString buffer

ExpandString correctly loops on the syscall until it reaches the
required buffer size but truncates it before converting it back to
string. The truncation limit is increased to 2^15 bytes which is the
documented maximum ExpandEnvironmentStrings output size.

This fixes TestExpandString on systems where len($PATH) > 1024.

Change-Id: I2a6f184eeca939121b458bcffe1a436a50f3298e
Reviewed-on: https://go-review.googlesource.com/9805


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>