Use naked functions

[jonathanpallant]

If we increase the MSRV to 1.88, we can use naked functions to replace many of the functions we have defined using pure assembly.

[thejpster]

We can't use Naked Functions because on Armv4T targets (armv4t-none-eabi and thumbv4t-none-eabi) calling a naked function with the wrong instruction set doesn't insert a Thunk like it should (there is no BLX instruction so you need to do a BL to the thunk, a BX to yourself and then BL to the right place). Calling a global_asm! function does get you the Thunk.

[thejpster]

Also the targets don't have thumb_interworking set, so you cannot use naked functions written to run in the 'opposite' mode.

See #111 for my failed attempt

[thejpster]

All the targets now support thumb interworking (at least on nightly).

This is blocked on rust-lang/rust#151946, which went to LLVM as llvm/llvm-project#180358.