cmd/compile: correct alignment of atomic.Int64

cmd/compile: correct alignment of atomic.Int64

Same as CL 417555, but for cmd/compile.

Fixes #54220

Change-Id: I4cc6deaf0a87c952f636888b4ab73f81a44bfebd
Reviewed-on: https://go-review.googlesource.com/c/go/+/420975
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>