Remove superfluous MOVT in mbed_start_application/start_new_application

Author: ccli8

platform/mbed_application.c

@@ -125,8 +125,9 @@ __asm static void start_new_application(void *sp, void *pc)
 void start_new_application(void *sp, void *pc)
 {
     __asm volatile (
-        "movw   r2, #0      \n" // Fail to compile "mov r2, #0" with ARMC6. Replace with movw/movt.
-        "movt   r2, #0      \n"
+        "movw   r2, #0      \n" // Fail to compile "mov r2, #0" with ARMC6. Replace with MOVW.
+                                // We needn't "movt r2, #0" immediately following because MOVW
+                                // will zero-extend the 16-bit immediate.
         "msr    control, r2 \n" // Switch to main stack
         "mov    sp, %0      \n"
         "msr    primask, r2 \n" // Enable interrupts