Add compiler tests for gcc15

[hanno-becker]

[hanno-becker]

@mkannwischer Can you look at this?

[mkannwischer]

Have to wait for NixOS/nixpkgs#401948 to be merged.

[mkannwischer]

While gcc15 has been added in nixpkgs and should soon also be backported to 25.05, it is currently broken on Darwin. A fix is being worked on in NixOS/nixpkgs#411025.

[hanno-becker]

NixOS/nixpkgs#411025 seems to have fixed x86_64-Darwin only. I opened #1141 excluding Darwin.