board_misoc/build: use rv32 as target arg

The original rv64 argument was only to match the misoc counterpart.

artiq/firmware/libboard_misoc/build.rs

@@ -13,7 +13,7 @@ fn main() {
 
     println!("cargo:rerun-if-changed={}", vectors_path.to_str().unwrap());
     cc::Build::new()
-        .flag("--target=riscv64-unknown-elf")
+        .flag("--target=riscv32-unknown-elf")
         .file(vectors_path)
         .compile("vectors");
 }