esp_system: make the abort operation compatible with clang

Clang warns that the original code wouldn't have any effect:

    warning: indirection of non-volatile null pointer will be deleted,
             not trap [-Wnull-dereference]
    note: consider using __builtin_trap() or qualifying pointer
          with 'volatile'

__builtin_trap translates to 'break 1, 15' instruction on Xtensa,
which might be okay in this case. However to absolutely certainly not
break anything for GCC builds, add 'volatile' instead.

components/esp_system/panic.c

@@ -399,7 +399,7 @@ void IRAM_ATTR __attribute__((noreturn, no_sanitize_undefined)) panic_abort(cons
 #endif
 #endif
 
-    *((int *) 0) = 0; // NOLINT(clang-analyzer-core.NullDereference) should be an invalid operation on targets
+    *((volatile int *) 0) = 0; // NOLINT(clang-analyzer-core.NullDereference) should be an invalid operation on targets
     while (1);
 }