esp_system: fix instrprohibited panic backtrace regression

First PC should have been ignored in the case of InstProhibited panic
when printing backtrace.

components/esp_system/include/esp_debug_helpers.h

@@ -41,6 +41,7 @@ typedef struct {
     uint32_t pc;       /* PC of the current frame */
     uint32_t sp;       /* SP of the current frame */
     uint32_t next_pc;  /* PC of the current frame's caller */
+    const void *exc_frame;  /* Pointer to the full frame data structure, if applicable */
 } esp_backtrace_frame_t;
 
 /**