Fix asm::delay() variability due to alignment

Author: ianrrees

cortex-m/src/asm.rs

@@ -33,6 +33,10 @@ pub fn delay(cycles: u32) {
     let real_cycles = 1 + cycles / 2;
     unsafe {
         asm!(
+            // The `bne` on some cores (eg Cortex-M4) will take a different number of instructions
+            // depending on the alignment of the branch target.  Set the alignment of the top of the
+            // loop to prevent surprising timing changes when the alignment of the delay() changes.
+            ".p2align 3",
             // Use local labels to avoid R_ARM_THM_JUMP8 relocations which fail on thumbv6m.
             "1:",
             "subs {}, #1",