Updates the RISC-V port from JEP 422 by adding partial support of JVMCI.

This feature aims to allow using the Native Image RISC-V LLVM backend, which does not use JVMCI for code emission.

The goal of this issue would thus be to at least implement the subset of JVMCI for RISC-V required by Graal compilation up to the LLVM backend. It is an explicit non-goal to support GraalVM JIT mode.