Build the rust code with target-cpu=native

cxx/Makefile

@@ -8,7 +8,7 @@ libspir_cxx.a: spir.o ../target/release/libspiral_spir.a
 	ar r $@ $<
 
 ../target/release/libspiral_spir.a: $(wildcard ../src/*.rs)
-	cargo build --release
+	RUSTFLAGS="-C target-cpu=native" cargo build --release
 
 clean:
 	-rm -f libspir_cxx.a spir.o spir_test.o spir_test