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.