runtime: use internal/cpu variables in assembler code

Using internal/cpu variables has the benefit of avoiding false sharing
(as those are padded) and allows memory and cache usage for these variables
to be shared by multiple packages.

Change-Id: I2bf68d03091bf52b466cf689230d5d25d5950037
Reviewed-on: https://go-review.googlesource.com/126599


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>