I made a copy of the per-arch _CacheLineSize definitons when checking in
runtime/internal/atomic. Now that runtime/internal/sys is checked in,
we can use the definition there.

Change-Id: I7242f6b633e4164f033b67ff471416b9d71c64d2
Reviewed-on: https://go-review.googlesource.com/16847Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>