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/jit/src/debug.rs

@@ -97,6 +97,7 @@ fn ensure_supported_elf_format(bytes: &[u8]) -> Result<Endianness, Error> {
         EM_AARCH64 => (),
         EM_X86_64 => (),
         EM_S390 => (),
+        EM_RISCV => (),
         machine => {
             bail!("Unsupported ELF target machine: {:x}", machine);
         }