Mark pre_init as deprecated

Author: romancardenas

cortex-m-rt/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Mark `pre_init` as deprecated
 - Add `set_msplim` feature to conditionally set the MSPLIM register at device
   reset ([#580]).
 

cortex-m-rt/macros/src/lib.rs

@@ -628,6 +628,7 @@ pub fn interrupt(args: TokenStream, input: TokenStream) -> TokenStream {
 }
 
 #[proc_macro_attribute]
+#[deprecated(note = "Use core::arch::global_asm! to define the __pre_init function instead")]
 pub fn pre_init(args: TokenStream, input: TokenStream) -> TokenStream {
     let f = parse_macro_input!(input as ItemFn);