Rename a function to not collide

Functions with the same name in the same module will now collide
(just like they do in normal Rust code)

Author: jonas-schievink

cortex-m-rt/tests/compile-fail/interrupt-invalid.rs

@@ -7,7 +7,7 @@ extern crate panic_halt;
 use cortex_m_rt::{entry, interrupt};
 
 #[entry]
-fn foo() -> ! {
+fn entry() -> ! {
     loop {}
 }