Change the sentinel value for __pre_init to something other than 1

The sentinel value of 0, while nice, was being optimized out as the compiler
assumed that the function pointer would never be 0.

Signed-off-by: Gabriel Smith <[email protected]>

Author: yodaldevoid

cortex-m-rt/link.x.in

@@ -54,7 +54,7 @@ EXTERN(__INTERRUPTS); /* `static` variable similar to `__EXCEPTIONS` */
 /* # Pre-initialization function */
 /* If the user overrides this using the `pre_init!` macro or by creating a `__pre_init` function,
    then the function this points to will be called before the RAM is initialized. */
-PROVIDE(__pre_init = 0);
+PROVIDE(__pre_init = 1);
 
 /* # Sections */
 SECTIONS

cortex-m-rt/src/lib.rs

@@ -491,7 +491,7 @@ pub unsafe extern "C" fn Reset() -> ! {
     }
 
     let pre_init: unsafe extern "C" fn() = __pre_init;
-    if pre_init as usize != 0 {
+    if pre_init as usize != 1 {
         pre_init();
     }