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>

cranelift/filetests/filetests/runtests/i128-arithmetic.clif

@@ -4,6 +4,7 @@ set enable_llvm_abi_extensions=true
 target aarch64
 target s390x
 target x86_64
+target riscv64
 
 function %add_i128(i128, i128) -> i128 {
 block0(v0: i128,v1: i128):