riscv: build: make cfg variables more robust

Change which Cargo `CFG` environment variables are used to select the
custom `cfg` variables.

See
rust-embedded#204 (comment)
for discussion.

riscv/CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 - Made `asm::wfi`, `fence`, `fence_i` and `sfence` safe (ie, removed `unsafe` from their definitions)
+- Made `cfg` variable selection more robust for custom targets
 
 ## [v0.11.0] - 2024-01-14
 

riscv/build.rs

@@ -1,12 +1,12 @@
 use std::env;
 
 fn main() {
-    let target = env::var("TARGET").unwrap();
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
 
-    if target.starts_with("riscv32") {
+    if target_arch == "riscv32" {
         println!("cargo:rustc-cfg=riscv");
         println!("cargo:rustc-cfg=riscv32");
-    } else if target.starts_with("riscv64") {
+    } else if target_arch == "riscv64" {
         println!("cargo:rustc-cfg=riscv");
         println!("cargo:rustc-cfg=riscv64");
     }