Add GCC atomic support (-moutline-atomics) for arm64 on Focal

[Salvatore]

Hi,
we recorded more than 30% performance regression on Ubuntu Focal for AWS Graviton instances since Nginx package is not compiled with "-moutline-atomics" cflag for arm64 architecture (337548 rps with default package and 484453 rps using the proposed flag).

As far as I know, only Ubuntu 20.04 is affected by it and we are proposing a fix for it (https://code.launchpad.net/~dipietro-salvatore/ubuntu/+source/nginx/+git/nginx/+merge/434825).

Set up to reproduce the issue:

- AWS instance: m6g.metal
- AWS ami: ami-0aa916c7b0be51092

- lsb_release -rd:
Description: Ubuntu 20.04.6 LTS
Release: 20.04

- apt-cache policy nginx:
nginx:
  Installed: (none)
  Candidate: 1.18.0-0ubuntu1.4
  Version table:
     1.18.0-0ubuntu1.4 500
        500 http://us-west-2.ec2.ports.ubuntu.com/ubuntu-ports focal-updates/main arm64 Packages
        500 http://ports.ubuntu.com/ubuntu-ports focal-security/main arm64 Packages
     1.17.10-0ubuntu1 500
        500 http://us-west-2.ec2.ports.ubuntu.com/ubuntu-ports focal/main arm64 Packages

[Christian Ehrhardt  (paelzer)]

I talked with Fabio (now subscribed), he will track and ensure this is done together with all the other "enable -moutline-atomics" cases.