Avoid direct casts from function to integer

[jannic]

Current nightly warns about direct casts from function to integer:

warning: direct cast of function item into an integer
   --> .../rp235x-hal/src/multicore.rs:371:36
    |
371 |                 core1_startup::<F> as usize,
    |                                    ^^^^^^^^
    |
    = note: `#[warn(function_casts_as_integer)]` on by default
help: first cast to a pointer `as *const ()`
    |
371 |                 core1_startup::<F> as *const () as usize,
    |                                    ++++++++++++

In our case the cast is intended, so apply the suggested fix.

As an alternative to *const () for the intermediate step, one could also use the actual fn type. However, in this case, that would be a rather long signature:
core1_startup::<F> as extern "C" fn(u64, u64, *mut core::mem::ManuallyDrop<F>, *mut usize) -> ! as usize

So I think here it's better to use the *const () shortcut.

[9names]

LGTM. Not sure why the cargo hack CI step failed, looks like just a spurious network error?

[jannic]

Very likely yes. I tried it locally, I do get valid gzip compressed data from curl -sSL https://github.com/taiki-e/cargo-hack/releases/download/v0.6.17/cargo-hack-x86_64-unknown-linux-gnu.tar.gz.