Expand description
Replacements for core::intrinsics to work around https://github.com/immunant/c2rust/issues/372
They are currently only define on riscv32; other platforms (or functions) can be added when necessary. They can’t be defined for all platforms, as Cortex-M0 don’t have this operation. (But then again, they’re not needed, as theire RIOT C code can’t contain these operations).