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/fiber/src/unix.rs

@@ -189,7 +189,9 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_arch = "s390x")] {
         // currently `global_asm!` isn't stable on s390x so this is an external
         // assembler file built with the `build.rs`.
-    } else {
+    } else if #[cfg(target_arch = "riscv64")]  {
+        mod riscv64;
+    }else {
         compile_error!("fibers are not supported on this CPU architecture");
     }
 }