cpu/cortexm_common: only set naked attribute on DEVELHELP hardfault h…

…andler

Fixes error on LLVM/Clang:

   cpu/cortexm_common/vectors_cortexm.c:287:5: error: non-ASM statement in naked function is not supported
       core_panic(PANIC_HARD_FAULT, "HARD FAULT HANDLER");
       ^
   cpu/cortexm_common/include/vectors_cortexm.h:65:46: note: attribute is here
   void hard_fault_default(void) __attribute__((naked));
                                                ^
   1 error generated.

cpu/cortexm_common/include/vectors_cortexm.h

@@ -62,7 +62,7 @@ void nmi_default(void);
  * causes of hard faults are access to un-aligned pointers on Cortex-M0 CPUs
  * and calls of function pointers that are set to NULL.
  */
-void hard_fault_default(void) __attribute__((naked));
+void hard_fault_default(void);
 
 /* The following four exceptions are only present for Cortex-M3 and -M4 CPUs */
 #if defined(CPU_ARCH_CORTEX_M3) || defined(CPU_ARCH_CORTEX_M4) || \

cpu/cortexm_common/vectors_cortexm.c

@@ -134,7 +134,7 @@ static inline int _stack_size_left(uint32_t required)
 }
 
 /* Trampoline function to save stack pointer before calling hard fault handler */
-void hard_fault_default(void)
+__attribute__((naked)) void hard_fault_default(void)
 {
     /* Get stack pointer where exception stack frame lies */
     __ASM volatile