How much pain is it to move everyone to a new GCC?

[remexre]

GCC 15 adds a few nice things:

• __builtin_stdc_rotate_left and __builtin_stdc_rotate_right, letting us remove our manual implementations in src/kernel/include/compiler.h, which don't actually work on e.g. _BitInt types.
• The counted_by attribute, which improves our ability to bounds-check things. See https://people.kernel.org/gustavoars/how-to-use-the-new-counted_by-attribute-in-c-and-linux for more on this.

How much does it suck for people using the devcontainer if we wanna force everyone to update? We could probably give a good error in src/kernel/include/compiler.h, at least.