Allow globals in reclaimed bluetooth memory (#2455)

libs/core/pxt.h

@@ -54,12 +54,22 @@ static inline int max_(int a, int b) {
         return b;
 }
 
+void initMicrobitGC();
+
 } // namespace pxt
 
 using namespace pxt;
 
 #define DEVICE_EVT_ANY 0
 
+#undef PXT_MAIN
+#define PXT_MAIN                                                                                   \
+    int main() {                                                                                   \
+        pxt::initMicrobitGC();                                                                     \
+        pxt::start();                                                                              \
+        return 0;                                                                                  \
+    }
+
 #endif
 
 // vim: ts=2 sw=2 expandtab