debug: Wrap rb_backtrace with HAVE_RB_BACKTRACE instead of CPU_ARM

Basically, preparation for a non-ARM backtrace support.

Change-Id: Icfd09fbc65a98f859e2a19f8d1111827a262a969

firmware/export/system.h

@@ -253,6 +253,11 @@ static inline void cpu_boost_unlock(void)
     #define MIN_STACK_ALIGN 8
 #endif
 
+/* Define this if target has support for generating backtraces */
+#ifdef CPU_ARM
+    #define HAVE_RB_BACKTRACE
+#endif
+
 #ifndef MIN_STACK_ALIGN
 #define MIN_STACK_ALIGN (sizeof (uintptr_t))
 #endif

firmware/panic.c

@@ -114,7 +114,7 @@ void panicf( const char *fmt, ...)
         }
     }
 
-#if defined(CPU_ARM)
+#if defined(HAVE_RB_BACKTRACE)
     rb_backtrace(pc, sp, &y);
 #endif
 #ifdef ROCKBOX_HAS_LOGF