Add ability to override number of IRQ vectors

[will-v-pi]

Adds PICO_VECTOR_TABLE_NUM_IRQS define to allow overriding the number of IRQ vectors in the vector table. This is useful to reduce the size of the vector table when not using some IRQs.

For example, if no IRQs are being used then you can add this to your CMakeLists.txt to reduce the size of the vector table in the binary

target_compile_definitions(my_executable PRIVATE PICO_VECTOR_TABLE_NUM_IRQS=0)

[kilograham]

Needs a PICO_CONFIG comment

[kilograham]

Note i was going to suggest we say you cannot use the IRQ at all, which is because there will be garbage in the table which will confuse set_exclusive_irq_handler or do the wrong thing for add_shared_irq_handler

That said perhaps instead we should (as part of this fix) change runtime_init_install_ram_vector_table() to copy the provided amount of IRQs and clear the rest

[will-v-pi]

I've pushed a change which solves this in a different way, by setting the PICO_RAM_VECTOR_TABLE_SIZE correctly based on PICO_VECTOR_TABLE_NUM_IRQS, and then checking against PICO_VECTOR_TABLE_NUM_IRQS in check_irq_param - I think this should prevent use of any IRQs which don't have vectors in the vector table?

[lurch]

check_irq_param uses invalid_params_if which I think means that it'll become a no-op in release builds? (but I dunno if that's a problem or not)

[will-v-pi]

Actually, yeah, I think upon further thought what I actually want is a way to disable all IRQ vectors, to create a minimal vector table. I'll submit a new PR with that

[kilograham]

think we need to be clear on what we're trying to achieve.

1. If the goal is to minimize the stored binary size, we note that we have space for a ROM vector table most of which (though the user can provide them at build time) is full of unused slots, and could choose not to store all the vtable entries. This would allow for (with RAM vector table) use of full set of IRQs at runtime at the cost of a full size RAM vector table
2. If the goal is to minimize both the stored binary size and the RAM usage at runtime and you don't ever plan to use IRQ number > N then we can likely clip both

Note this PR implements 2... we just need to figure if 1 is ever that interesting and if so, either do that too, or at least think how it would play with 2.

[will-v-pi]

Superceded by #2233