arch: arm: fix USE_SWITCH for IAR

orr fix is as reported in review:
```
The add causes a crash with IAR tools as the address loaded to r8
already has the lowest bit set, and the add causes it to be set to ARM
mode. The orr instruction works fine with both scenarios
```

`UDF 0` seems to break on IAR but `UDF #0` works for all.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>

Author: wearyzen

arch/arm/core/cortex_m/arm-m-switch.c

@@ -223,7 +223,7 @@ static bool fpu_state_pushed(uint32_t lr)
  */
 __attribute__((naked)) void arm_m_iciit_stub(void)
 {
-	__asm__("udf 0;");
+	__asm__("udf #0;");
 }
 
 /* Called out of interrupt entry to test for an interrupted instruction */

include/zephyr/arch/arm/arm-m-switch.h

@@ -270,7 +270,7 @@ static ALWAYS_INLINE void arm_m_switch(void *switch_to, void **switched_from)
 			 "mov r6, r12;"
 			 "mov r7, lr;"
 			 "ldr r8, =3f;"    /* address of restore PC */
-			 "add r8, r8, #1;" /* set thumb bit */
+			 "orr r8, r8, #1;" /* set thumb bit */
 			 "push {r6-r8};"
 			 "sub sp, sp, #24;" /* skip over space for r6-r11 */
 			 "push {r0-r5};"