Fix start_new_application compile error on Cortex-M23

With "mov r2, #0", compile OK with GCC_ARM, but failed with ARMC6.
With "ldr r2, =0", compile OK with ARMC6, but failed with GCC_ARM.
Finally, with "movw r2, #0"/"movt r2, #0", compile OK with both ARMC6 and GCC_ARM.

Author: ccli8

platform/mbed_application.c

@@ -117,20 +117,13 @@ __asm static void start_new_application(void *sp, void *pc)
 
 void start_new_application(void *sp, void *pc)
 {
-#if defined(__CORTEX_M23)
     __asm volatile (
-        "ldr    r2, =0      \n"
-        "msr    control, r2 \n" // Switch to main stack
-        "mov    sp, %0      \n"
-        "msr    primask, r2 \n" // Enable interrupts
-        "bx     %1          \n"
-        :
-        : "l" (sp), "l" (pc)
-        : "r2", "cc", "memory"
-    );
+#if defined(__CORTEX_M23)
+        "movw   r2, #0      \n"
+        "movt   r2, #0      \n"
 #else
-    __asm volatile (
         "mov    r2, #0      \n"
+#endif
         "msr    control, r2 \n" // Switch to main stack
         "mov    sp, %0      \n"
         "msr    primask, r2 \n" // Enable interrupts
@@ -139,7 +132,6 @@ void start_new_application(void *sp, void *pc)
         : "l" (sp), "l" (pc)
         : "r2", "cc", "memory"
     );
-#endif
 }
 
 #else