add riscv64 backend for cranelift. (#4271)

Add a RISC-V 64 (`riscv64`, RV64GC) backend.

Co-authored-by: yuyang <756445638@qq.com>
Co-authored-by: Chris Fallin <chris@cfallin.org>
Co-authored-by: Afonso Bordado <afonsobordado@az8.co>

crates/environ/src/compilation.rs

@@ -240,6 +240,7 @@ pub trait Compiler: Send + Sync {
                 Arm(_) => Architecture::Arm,
                 Aarch64(_) => Architecture::Aarch64,
                 S390x => Architecture::S390x,
+                Riscv64(_) => Architecture::Riscv64,
                 architecture => {
                     anyhow::bail!("target architecture {:?} is unsupported", architecture,);
                 }