riscv: build: make cfg variables more robust

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

See
#204 (comment)
for discussion.

riscv/build.rs

@@ -1,12 +1,13 @@
 use std::env;
 
 fn main() {
-    let target = env::var("TARGET").unwrap();
+    let target = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+    let ptr_width = env::var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap();
 
-    if target.starts_with("riscv32") {
+    if target.contains("riscv") && ptr_width == "32" {
         println!("cargo:rustc-cfg=riscv");
         println!("cargo:rustc-cfg=riscv32");
-    } else if target.starts_with("riscv64") {
+    } else if target.contains("riscv") && ptr_width == "64" {
         println!("cargo:rustc-cfg=riscv");
         println!("cargo:rustc-cfg=riscv64");
     }