Support interrupt vector number in `avr_interrupt` and `avr_signal` calling conventions

[alexrp]

GCC 15 added support for this: https://gcc.gnu.org/gcc-15/changes.html#avr

Obviously we can't do much with it on the backend until LLVM supports it, or we have a self-hosted AVR backend.

[alexrp]

We may also want to support the newly added noblock attribute as part of this.