[Cortex] Add preliminary support for core-dependent defines ARMv6m / ARMv7m, ARMv7em

lib/cm3/sync.c

@@ -19,8 +19,15 @@
 
 #include <libopencm3/cm3/sync.h>
 
-#if defined(LPC43XX_M0)
-#warning "Currently sync is not supported on Cortex-M0"
+/* DMB is supported on CM0 */
+void __dmb()
+{
+	__asm__ volatile ("dmb");
+}
+
+/* Those are defined only on CM3 or CM4 */
+#if defined(__ARM_ARCH_6M__)
+#warning "sync not supported on ARMv6-M arch"
 #else
 
 uint32_t __ldrex(volatile uint32_t *addr)
@@ -38,11 +45,6 @@ uint32_t __strex(uint32_t val, volatile uint32_t *addr)
 	return res;
 }
 
-void __dmb()
-{
-	__asm__ volatile ("dmb");
-}
-
 void mutex_lock(mutex_t *m)
 {
 	uint32_t status = 0;