Auto merge of #125419 - GuillaumeGomez:add-gcc-to-dist, r=Kobzol

[bootstrap] Add support for building gcc and libgccjit

As `@eholk` summarized below:

> From my understanding, this change would add libgccjit as an optional component to the Rust distribution. This library is licensed under GPLv2 and currently we do not have any other components under that license so it would be a new license, and one that is generally more restrictive than the other licenses we use.

It'll greatly improve the experience for anyone wanting to work on the GCC backend from the compiler.

Should help with rust-lang/rust#124172.
Will unblock #124353.

r? `@Kobzol`