feat: add vcore_debug function

This does the same as core_debug, but (like printf vs vprintf) accepts
an already processed va_list to allow other vararg functions to forward
their argument lists to this function.

Author: matthijskooijman

Diff for: cores/arduino/core_debug.c

@@ -3,3 +3,4 @@
 // Ensure inline functions have a definition emitted for when they are
 // not inlined (needed for C functions only)
 extern void core_debug(const char *format, ...);
+extern void vcore_debug(const char *format, va_list args);

Diff for: cores/arduino/core_debug.h

@@ -28,6 +28,16 @@ inline void core_debug(const char *format, ...)
 #endif /* NDEBUG */
 }
 
+inline void vcore_debug(const char *format, va_list args)
+{
+#if !defined(NDEBUG)
+  vfprintf(stderr, format, args);
+#else
+  (void)(format);
+  (void)(args);
+#endif /* NDEBUG */
+}
+
 #ifdef __cplusplus
 }
 #endif