features: adds rt feature for gd32vf103-pac/rt

[rmsyn]

Adds a rt feature to allow users to opt-in to using the gd32vf103-pac/rt feature. This exposes the interrupt macro, allowing users to define functions for system interrupts.

This is preferable to using inline assembly to include the code in eclic-mode-hack.S which causes symbol collisions with user-defined interrupt functions, and strange bugs for handler execution.

[Disasm]

No inline assembly is needed to define an interrupt handler: see https://github.com/riscv-rust/longan-nano/blob/master/examples/interrupt.rs for example. But using the macro is indeed a bit safer way of declaring an interrupt handler. eclic-mode-hack.S is still needed though since it implements interrupt dispatch. Without this code the interrupt macro just defines a function which will not be called.

[rmsyn]

Without this code the interrupt macro just defines a function which will not be called.

AFAICT this code isn't actually linked into the binary anywhere. I removed the code entirely, built against the gd32vf103-pac::interrupt macro, and the modified https://github.com/riscv-rust/longan-nano/blob/master/examples/interrupt.rs example still works the same. I think it is completely safe to remove eclic-mode-hack.S, assemble.sh, and gen-intr-lut.sh.

Also, actually moving the eclic-mode-hack.S code into inline assembly, which ensures that the code is included in linked library code, breaks the longan-nano interrupt example (without modification to the example). Meaning, actually using the code in eclic-mode-hack.S breaks existing working code.

[Disasm]

It gets linked into the code by build.rs of this crate. eclic-mode-hack.S comes pre-compiled as bin/gd32vf103xx-hal.a. That's why maybe it works when you remove the code, try removing build.rs as well.

[rmsyn]

It gets linked into the code by build.rs of this crate. eclic-mode-hack.S comes pre-compiled as bin/gd32vf103xx-hal.a.

100% missed the precomp thing... I ended up removing build.rs, bin, and the like, compilation fails.

I have some current WIP stuff to move it to Rust here: #55

Not needed for this PR, though. Thanks for the pointers on how stuff gets linked together.

[Disasm]

In any case, thank you for the PR!

[Disasm]

Do you mind adding "rt" feature also to the GitHub Actions rules? https://github.com/riscv-rust/gd32vf103xx-hal/blob/master/.github/workflows/ci.yml#L26

[Disasm]

Thanks!