Mark main() with a C ABI

The documentation says that main() should have a C ABI. Additionally,
the #[entry] macro will create a main() function that has a C ABI.
Marking main() as 'extern "Rust"' is inconsistent with its calling
convention.

Author: mciantyre

cortex-m-rt/src/lib.rs

@@ -897,12 +897,11 @@ pub unsafe extern "C" fn Reset() -> ! {
         static mut __edata: u32;
         static __sidata: u32;
 
-    }
-
-    extern "Rust" {
         // This symbol will be provided by the user via `#[entry]`
         fn main() -> !;
+    }
 
+    extern "Rust" {
         // This symbol will be provided by the user via `#[pre_init]`
         fn __pre_init();
     }