[arch][arm][cortex-m] select the simplified arch interrupt save/resto…

…re for armv6m too

Previously had only selected armv7m for the simplified version.
Forgot that cortex-m0s are armv6m and could use this version.

arch/arm/include/arch/spinlock.h

@@ -50,7 +50,7 @@ static inline void arch_spin_unlock(spin_lock_t *lock) {
 
 #endif
 
-#if !ARM_ISA_ARMV7M
+#if !(ARM_ISA_ARMV7M || ARM_ISA_ARMV6M)
 
 /* ARM specific flags */
 #define SPIN_LOCK_FLAG_IRQ                      0x40000000