Replace custom `#[entry]` spinlock code with pre-init function

[thejpster]

We use custom code in the #[entry] macro to unlock the spinlocks on start-up. But, if you use rtic, you don't use macro and the spinlocks can remain locked (see knurling-rs/defmt#975 for real-world consequences).

embassy rp-rs uses a _pre_init function to clear all the locks, which is executed by cortex-m-rt before main and regardless of which mechanism you use to get to main. See https://github.com/embassy-rs/embassy/blob/main/embassy-rp/src/lib.rs#L633. However, a pre-init function written in Rust is UB, so we should probably write it in assembly language.