internal/cpu: add a CacheLinePadSize constant

The new constant CacheLinePadSize can be used to compute best effort
alignment of structs to cache lines.

e.g. the runtime can use this in the locktab definition:
var locktab [57]struct {
        l   spinlock
        pad [cpu.CacheLinePadSize - unsafe.Sizeof(spinlock{})]byte
}

Change-Id: I86f6fbfc5ee7436f742776a7d4a99a1d54ffccc8
Reviewed-on: https://go-review.googlesource.com/131237


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>