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/runtime/src/trampolines.rs

@@ -51,6 +51,9 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_arch = "s390x")] {
         #[macro_use]
         mod s390x;
+    }else if #[cfg(target_arch = "riscv64")] {
+        #[macro_use]
+        mod riscv64;
     } else {
         compile_error!("unsupported architecture");
     }