WIP: [Linux] Add Linux sysroot scripts

[cbracken]

trafficstars

This is an unmodified (other than formatting) copy of the Chromium sysroot scripts found at: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/build/linux/sysroot_scripts/

The instructions in README.md have been added, copied from: https://chromium.googlesource.com/chromium/src/+/master/docs/linux/sysroot.md

These will need to be modified to build to Flutter's cloud storage/CIPD.

Issue: https://github.com/flutter/flutter/issues/139110

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. You must list at least one issue.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the CLA.
• [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[goderbauer]

(triage): I spoke to @cbracken last week and he said this is still on his radar.

[goderbauer]

(triage): I spoke to @cbracken last week and he said this is still on his radar.

[Piinks]

(PR Triage): @cbracken is this still on your radar? :)