Add php-src mirror and use gmp mirror site

[crazywhalecc]

What does this PR do?

• Add php.net mirror site
• Use ftp.gnu.org instead of gmplib.org (GitHub IP is banned)
• Closes implement fallback download for php-src #1047

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
If a modification is not involved, please skip it directly.

• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[henderkes]

I unfortunately think the mirroring isn't going to be enough, because the site sometimes goes down entirely. I was thinking of a mirror that actually mirrors the entire data, not just reverse proxies to php.net (unless that's what you do?). Could use github releases instead, albeit not optimal, it's reachable at least.

[crazywhalecc]

Of course it's full mirror. Using standalone server I mentioned in discord and php official rsync:

5 * * * * rsync -avzC --timeout=600 --delete --delete-after  rsync.php.net::phpweb /var/www/phpmirror.static-php.dev/

This server looks enough to host all of our services.