test: enable regabi test on arm64

CL 324890 turned on register ABI by default on ARM64, causing neither
live.go nor live_regabi.go is run on ARM64.

This CL enables live_regabi.go test for ARM64.

Change-Id: I0c483a38b761c5a6f1fa9a5b3324b5da64907e61
Reviewed-on: https://go-review.googlesource.com/c/go/+/343531


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>