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/fuzzing/src/oracles.rs

@@ -31,7 +31,7 @@ use std::time::{Duration, Instant};
 use wasmtime::*;
 use wasmtime_wast::WastContext;
 
-#[cfg(not(any(windows, target_arch = "s390x")))]
+#[cfg(not(any(windows, target_arch = "s390x", target_arch = "riscv64")))]
 mod diff_v8;
 
 static CNT: AtomicUsize = AtomicUsize::new(0);