runtime/internal/math: add Add64

runtime/internal/math: add Add64

This makes the intrinsic available on 64-bit platforms,
since the runtime cannot import math/bits.

Change-Id: I5296cc6a97d1cb4756ab369d96dc9605df9f8247
Reviewed-on: https://go-review.googlesource.com/c/go/+/516861
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>