Remove weak symbols, use abort for defaults.

[rslawson]

As per #247, using weak symbols for overridable functions has proven problematic with lto = true. This is a partial reversion of that, though there are still other functions this may yet be done with. There is also another solution to be done as a sort of nightly-only thing with naked functions and #[linkage = "weak"], however I have opted not to go with that seeing as this project doesn't currently require nightly and I don't think this is an issue worth changing that over.

[rslawson]

I think there may be something else that I need to do to the linker script, since as it is now it currently fails to build on one of the projects I'm working on with the error

  = note: rust-lld: error: 
          BUG(riscv-rt): start of .heap is not 4-byte aligned
          
          rust-lld: error: 
          BUG(riscv-rt): start of .heap is not 4-byte aligned

[romancardenas]

Try to rebase from master. Build errors should disappear

[romancardenas]

@rslawson please, add a brief note in README.md describing your changes

[rslawson]

Yep, they did disappear. I'll make the change to README.md now. Are there any other symbols I should give the same treatment, or is it enough just to change DefaultHandler and ExceptionHandler?

[romancardenas]

LGTM.

I would like to discuss this work with @rust-embedded/all in the next meeting in case there is a better approach to deal with weak symbols and LTO optimizations in Rust.

[rslawson]

In the meantime I'll try to keep this rebased with an up-to-date master branch from this repo.

[romancardenas]

We discussed this today and... we didn't come up with a better solution.

Let's leave this PR and #247 open to keep discussing what to do with weak symbols. We must:

1. Enumerate all the weak symbols we are currently using in riscv-rt
2. Propose an alternative methodology for them. Not all symbols are easily replaceable as abort or interrupt handlers (e.g., mp_hook).

@rslawson , would you be happy to keep working on this PR until we decide what to do with the remaining weak symbols?

[rslawson]

Sure thing, I can do that.

[rmsyn]

As per #247, using weak symbols for overridable functions has proven problematic with lto = true.

Is this only a problem with rust-lld (read lld)? If so, does using another linker with LTO resolve the issue you're experiencing @rslawson?

Maybe until there is a stable solution for weak linkage in Rust, we could recommend using a different linker instead of introducing nightly-only features?