Implement coprocessor access instructions

[spcan]

Implements coprocessor basic access instructions (MCR, MRC, MCRR, MRRC) that compile to a single instruction under --release profiles

[spcan]

CI tests:

• rustfmt fails but functions are formatted the same as the rest of the assembly functions
• stm32 target tests fail due to 3 probes being present: Need to repeat test

[adamgreig]

Thanks for the RP! What's the longer term plan, since these methods are all pub(crate)? Do you imagine adding further coprocessor-related functionality in cortex-m? Should the methods be publicly available?

[spcan]

The aim of these is to expose a common method to interact through the ARM coprocessor interface. Separate libs can then implement the coprocessor driver's using these functions.

For example, I'm using it currently to interact with the LPC55S69 math coprocessor

[adamgreig]

If you want separate libs to later use these functions, don't they need to be pub instead of pub(crate)?

[spcan]

Yeah, good catch, that was there on the test library, forgot to remove it

[spcan]

Ok, it's now failing to compile because const ASM operands are unstable, needing #![feature(asm_const)], can this be enabled in the crate, or should this PR wait until ASM const is stabilized??

This is getting stabilized soon as per #93332