Add a CacheLinePad struct type that has a size of cacheLineSize.
This can be used for padding structs in order to avoid false sharing.

Fixes golang/go#25203

Change-Id: I6b6daf91ea0e9c50428ffa053035429c3921b461
Reviewed-on: https://go-review.googlesource.com/111775
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>