runtime: use FP offsets for pipe/pipe2 on freebsd/arm64 and linux/arm64

This is more readable and less error-prone than using RSP offsets.

Suggested during review of CL 212765.

Change-Id: I070190abeeac8eae5dbd414407602619d9d57422
Reviewed-on: https://go-review.googlesource.com/c/go/+/213577


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>