riscv: build: use cfg-check directives in build script

Uses new lint checks for `cfg` variables.

Author: rmsyn

riscv/build.rs

@@ -1,6 +1,10 @@
 use std::env;
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(riscv)");
+    println!("cargo:rustc-check-cfg=cfg(riscv32)");
+    println!("cargo:rustc-check-cfg=cfg(riscv64)");
+
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
 
     if target_arch == "riscv32" {