py: Make struct-initializing macros compatible with C++.

This requires explicitly naming and initializing all members so add that
where needed and possible.  For MP_DEFINE_NLR_JUMP_CALLBACK_FUNCTION_1
this would require initializing the .callback member, but that's a bit
of a waste since the macro is always followed by a call to
nlr_push_jump_callback() to initialize exactly that member, so rewrite
the macro without initializers.

Signed-off-by: stijn <stijn@ignitron.net>

py/runtime.h

@@ -30,12 +30,12 @@
 #include "py/pystack.h"
 #include "py/cstack.h"
 
-// For use with mp_call_function_1_from_nlr_jump_callback.
+// Initialize an nlr_jump_callback_node_call_function_1_t struct for use with
+// nlr_push_jump_callback(&ctx.callback, mp_call_function_1_from_nlr_jump_callback);
 #define MP_DEFINE_NLR_JUMP_CALLBACK_FUNCTION_1(ctx, f, a) \
-    nlr_jump_callback_node_call_function_1_t ctx = { \
-        .func = (void (*)(void *))(f), \
-        .arg = (a), \
-    }
+    nlr_jump_callback_node_call_function_1_t ctx; \
+    ctx.func = (void (*)(void *))(f); \
+    ctx.arg = (a)
 
 typedef enum {
     MP_VM_RETURN_NORMAL,