Add RISC-V 64 bit (microsoft#4894)

- Add riscv64 target to config.guess
- Update config-ix.cmake to update config-ix.cmake to support riscv64
- Tested on riscv64

Author: FantasqueX

autoconf/config.guess

@@ -977,6 +977,9 @@ EOF
     ppc:Linux:*:*)
 	echo powerpc-unknown-linux-gnu
 	exit ;;
+    riscv64:Linux:*:*)
+	echo ${UNAME_MACHINE}-unknown-linux-gnu
+	exit ;;
     s390:Linux:*:* | s390x:Linux:*:*)
 	echo ${UNAME_MACHINE}-ibm-linux
 	exit ;;

cmake/config-ix.cmake

@@ -365,6 +365,8 @@ elseif (LLVM_NATIVE_ARCH MATCHES "wasm32")
   set(LLVM_NATIVE_ARCH WebAssembly)
 elseif (LLVM_NATIVE_ARCH MATCHES "wasm64")
   set(LLVM_NATIVE_ARCH WebAssembly)
+elseif (LLVM_NATIVE_ARCH MATCHES "riscv64")
+  set(LLVM_NATIVE_ARCH RISCV)
 else ()
   message(FATAL_ERROR "Unknown architecture ${LLVM_NATIVE_ARCH}")
 endif ()