feat(ext/node): add riscv64 in process.arch (#23016)

Rust triple for riscv64 is riscv64gc. Although there are no official
builds for architectures other than x86_64 and aarch64, Arch Linux
RISC-V has managed to package Deno on riscv64:
https://github.com/felixonmars/archriscv-packages/blob/master/deno/riscv64.patch

Ref: #18702

ext/node/polyfills/_process/process.ts

@@ -19,6 +19,8 @@ export function arch(): string {
     return "x64";
   } else if (core.build.arch == "aarch64") {
     return "arm64";
+  } else if (core.build.arch == "riscv64gc") {
+    return "riscv64";
   } else {
     throw Error("unreachable");
   }