This is the really cool part, the ability to add custom instructions are part of the spec which makes them work through standard compilers. Any compiler that can build a RISC-V program can handle the custom instructions as inline assembly so all one needs to do is write a library to abstract them. Obviously you don't have the off-the-shelf ability for the compiler to insert the instructions automatically, but this is rarely the goal for small-to-medium projects.

Example:

https://nitish2112.github.io/post/adding-instruction-riscv/