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/wasmtime/src/engine.rs

@@ -490,6 +490,17 @@ impl Engine {
             }
         }
 
+        #[cfg(target_arch = "riscv64")]
+        {
+            enabled = match flag {
+                // make sure `test_isa_flags_mismatch` test pass.
+                "not_a_flag" => None,
+                // due to `is_riscv64_feature_detected` is not stable.
+                // we cannot use it.
+                _ => Some(true),
+            }
+        }
+
         #[cfg(target_arch = "x86_64")]
         {
             enabled = match flag {