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

Uses new lint checks for `cfg` variables.

riscv-rt/CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Moved all the assembly code to `asm.rs`
 - Use `weak` symbols for functions such as `_mp_hook` or `_start_trap`
 - `abort` is now `weak`, so it is possible to link third-party libraries including this symbol.
+- Made `cfg` variable selection more robust for custom targets
 
 ### Removed
 

riscv-rt/build.rs

@@ -72,14 +72,23 @@ fn parse_target(target: &str, cargo_flags: &str) -> (u32, HashSet<char>) {
 }
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(riscv)");
+    println!("cargo:rustc-check-cfg=cfg(riscv32)");
+    println!("cargo:rustc-check-cfg=cfg(riscv64)");
+    println!("cargo:rustc-check-cfg=cfg(riscvi)");
+    println!("cargo:rustc-check-cfg=cfg(riscvm)");
+    println!("cargo:rustc-check-cfg=cfg(riscva)");
+    println!("cargo:rustc-check-cfg=cfg(riscvf)");
+    println!("cargo:rustc-check-cfg=cfg(riscvd)");
+    println!("cargo:rustc-check-cfg=cfg(riscvc)");
+
     let target = env::var("TARGET").unwrap();
     let cargo_flags = env::var("CARGO_ENCODED_RUSTFLAGS").unwrap();
     let _name = env::var("CARGO_PKG_NAME").unwrap();
 
     // set configuration flags depending on the target
     if target.starts_with("riscv") {
         println!("cargo:rustc-cfg=riscv");
-
         // This is required until target_arch & target_feature risc-v work is
         // stable and in-use (rust 1.75.0)
         let (bits, extensions) = parse_target(&target, &cargo_flags);