Rollup merge of #137340 - Kobzol:bootstrap-dir-check, r=onur-ozkan

Add a notice about missing GCC sources into source tarballs

I didn't use `.gitkeep`, because that would be a hidden file that might not be noticed, whereas we want to let people inspecting the archive know why the sources are missing.

Inspired by rust-lang/rust#137332.

r? `@onur-ozkan`

Author: workingjubilee