Change usage example for statically allocated heap

The linker needs to know how big the heap is, otherwise, it could place
some of the global variables in the heap region.

Author: nviennot

examples/global_alloc.rs

@@ -16,9 +16,12 @@ static ALLOCATOR: CortexMHeap = CortexMHeap::empty();
 #[entry]
 fn main() -> ! {
     // Initialize the allocator BEFORE you use it
-    let start = cortex_m_rt::heap_start() as usize;
-    let size = 1024; // in bytes
-    unsafe { ALLOCATOR.init(start, size) }
+    {
+        use core::mem::MaybeUninit;
+        const HEAP_SIZE: usize = 1024;
+        static mut HEAP: [MaybeUninit<u8>; HEAP_SIZE] = [MaybeUninit::uninit(); HEAP_SIZE];
+        unsafe { ALLOCATOR.init((&mut HEAP).as_ptr() as usize, HEAP_SIZE) }
+    }
 
     let mut xs = Vec::new();
     xs.push(1);