Consider replacing `#[pre_init]` with `global_asm!`

https://docs.rs/cortex-m-rt/0.7.5/cortex_m_rt/#features says:

A #[pre_init] macro is also provided to run a function before RAM initialisation, but its use is deprecated as it is not defined behaviour to execute Rust code before initialisation. It is still possible to create a custom pre_init function using assembly.

The macro has also been marked as deprecated in the upstream project. In https://github.com/oxidecomputer/hubris/pull/2571, I added a larger warning about this, but the main sticking point is that pre_init runs before memory is initialized, including zero-initialized .bss statics, as well as flash-copied .data statics. The Rust Abstract Machine requires that statics have been initialized prior to executing any Rust code, otherwise we would be able to observe uninitialized memory (in the form of statics) using only safe code.

Unfortunately, we DO need to run some custom code before the standard cortex-m-rt initialization code runs: we have to turn on the necessary RAM sections BEFORE they can be initialized! This is what the pre_init hook always existed to enable. Right now, we are very careful to avoid interacting with statics in this function, but it still leaves us in a somewhat grey area with regards to soundness, as we are running Rust code without statics being actually initialized.

The currently recommended behavior is to replace this "life before main" code with some form of assembly, as assembly does not have any soundness concerns of its own to worry about. As our current code only needs to peek and poke at a few registers, this should be fairly manageable.

Locate the current #[pre_init] hook and trace the startup entry point it uses before RAM initialization. Review how the hook enables RAM sections and accesses registers, then determine how that work can be represented with global_asm! while preserving initialization order and behavior.