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

Uses new lint checks for `cfg` variables.

Author: rmsyn

riscv-semihosting/CHANGELOG.md

@@ -5,6 +5,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Changed
+
+- Made `cfg` variable selection more robust for custom targets
+
 ## [v0.1.0] - 2023-01-18
 
 - Add recommendation for `semihosting` in README.md.

riscv-semihosting/build.rs

@@ -1,6 +1,8 @@
 use std::env;
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(riscv)");
+
     let target = env::var("TARGET").unwrap();
 
     if target.starts_with("riscv") {