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.