Added test for double attribute

Author: korken89

cortex-m-rt/tests/compile-fail/whitelist-double-attr.rs

@@ -0,0 +1,13 @@
+#![no_main]
+#![no_std]
+
+extern crate cortex_m_rt;
+extern crate panic_halt;
+
+use cortex_m_rt::{entry, exception};
+
+#[exception]
+#[entry] //~ ERROR this attribute is not allowed on a function controlled by cortex-m-rt
+fn SVCall() -> ! {
+    loop {}
+}