Allow replacement of functions with hardware-specific alternatives

[jonathanpallant]

The Raspberry Pi Silicon RP2040 is a dual-Cortex-M0+ based microcontroller, but it also has a Hardware Divide peripheral.

It it possible to mark __udivmodsi4 (and/or similar) as weak symbols so we can replace them in a library crate? Or is there some other supported way to drop-in a faster platform-specific divide algorithm?

Ref: rp-rs/rp-hal#278

[Amanieu]

I'm hesitant to involve weak symbols since they have some over on targets that use dynamic linking: defining a symbol as weak forces all calls to it to be indirect via the PLT since a strong definition could be present in a shared library that is loaded at runtime.

Instead we should prefer the current system of relying on the linker as described by rp-rs/rp-hal#278 (comment). Rustc guarantees that the compiler_builtins crate is always linked last so its symbols will only be used if no other definition is available in the libraries listed in the linker args.

There are currently a few issues with this due to the fact that compiler-builtins isn't putting each intrinsic in its own object file (#412), but this is something that should be fixed within compiler-builtins.