cpu: add basic support for GOARCH=sparc64

cpu.go uses cacheLineSize for CacheLinePad and calls initOptions, both
of which are supplied per architecture, so the package does not build on
sparc64. This adds those two for it.

Updates golang/go#55000

Change-Id: I71d6bbde37cda3e263dbb1c65177fa619494c6dc
GitHub-Last-Rev: ff36fd1f54176d099fc3577d0b7ae5557ba464f7
GitHub-Pull-Request: golang/sys#289
Reviewed-on: https://go-review.googlesource.com/c/sys/+/822844
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: David Chase <drchase@google.com>