Update README

[Ozaq]

Description

Now list actually tested compilers. Updated some fomatting and dropped references to dead links / AIX.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌦️ >> Documentation << 🌦️
https://sites.ecmwf.int/docs/dev-section/eckit/pull-requests/PR-261

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.25%. Comparing base (f912327) to head (d9382a9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #261   +/-   ##
========================================
  Coverage    66.25%   66.25%           
========================================
  Files         1130     1130           
  Lines        57939    57939           
  Branches      4393     4393           
========================================
  Hits         38390    38390           
  Misses       19549    19549           

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