test/codegen: codify bit related code generation for arm64

test/codegen: codify bit related code generation for arm64

Also more consistently include commas after constants to increase
accuracy (i.e. "1," cannot inadvertantly match "10")

Change-Id: I480a73859d2e83354b8e9f94bc73c6563976d0e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/733460
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>