[CMSIS] Fixed a Clang build warning by conditionally applying the GCC-specific optimize(Os) attribute in ExitRun0Mode()

Change-Id: Ib38dc35f39761ebb660d276f54f70f63727273ac

Include/system_stm32h7xx.h

diff --git a/Include/system_stm32h7xx.h b/Include/system_stm32h7xx.h
index cfb383d..2ae989b 100644
--- a/Include/system_stm32h7xx.h
+++ b/Include/system_stm32h7xx.h
@@ -84,7 +84,7 @@
 
 extern void SystemInit(void);
 extern void SystemCoreClockUpdate(void);
-#if defined(__GNUC__) && !defined(__ARMCC_VERSION)
+#if defined(__GNUC__) && !defined(__clang__) && !defined(__ARMCC_VERSION)
   extern void ExitRun0Mode(void) __attribute__((optimize("Os")));
 #else
   extern void ExitRun0Mode(void);