Fix .type directive syntax for ARM targets

[vak-leon]

ARM GNU assembler uses %function instead of @function for the .type directive. The @function syntax causes build failures on arm-unknown-linux-gnueabihf:

error: expected STT_<TYPE_IN_UPPER_CASE>, '#<type>', '%<type>' or "<type>"
 --> <inline asm>:2:15
  |
2 | .type _start, @function
  |               ^

This adds an ARM-specific variant of the naked_fn! macro that uses the correct syntax.

Tested with cargo build --target arm-unknown-linux-gnueabihf.

[bjorn3]

#[naked] is now available on stable.

[vak-leon]

Thanks for the pointer! Unfortunately the nightly feature forces the unwinding crate which conflicts with panic="abort" builds. The global_asm fallback path is still needed for minimal no_std binaries that don't want unwinding overhead.

[bjorn3]

I don't mean that you should enable the nightly feature. Rather I think you can change origin to use #[naked] rather than global_asm!() even the nightly feature is disabled.

[vak-leon]

Ah, you're right - I was confusing the nightly feature with #[naked] itself. Makes sense that stable #[naked] would handle the platform-specific asm syntax internally, avoiding this whole %function vs @function mess.

Happy to look into refactoring naked_fn! to use #[naked] directly if that's the way to go. Want me to update this PR or open a fresh one?

[bjorn3]

A new PR probably makes sense.

[vak-leon]

Closing in favor of #169 which takes the cleaner approach of using stable #[naked] directly.