Move linker flags to build script.

tests/riscv32/build.rs

@@ -0,0 +1,8 @@
+use std::error::Error;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    println!("cargo:rustc-link-arg-bins=-Tmemory.x");
+    println!("cargo:rustc-link-arg-bins=-Tlink.x");
+
+    Ok(())
+}