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

Updates #25203

Change-Id: Icb95ae68d3c711f5f8217140811cad1a1d5be79a
Reviewed-on: https://go-review.googlesource.com/116276
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>