Thunks: Support thunk init function on host

Allows library to have an additional initialization function

ThunkLibs/include/common/Host.h

@@ -14,6 +14,15 @@ typedef void fex_call_callback_t(uintptr_t callback, void *arg0, void* arg1);
 static fex_call_callback_t* call_guest;
 
 #define EXPORTS(name) extern "C" { ExportEntry* fexthunks_exports_##name(void *a0, uintptr_t a1) { call_guest = (fex_call_callback_t*)a0; fexldr_init_##name(); return exports; } }
+#define EXPORTS_INIT(name, init_fn) \
+  extern "C" { \
+    ExportEntry* fexthunks_exports_##name(void *a0, uintptr_t a1) { \
+      call_guest = (fex_call_callback_t*)a0; \
+      fexldr_init_##name(); \
+      init_fn (); \
+      return exports; \
+    } }
+
 #define EXPORTS_WITH_CALLBACKS(name) extern "C" { ExportEntry* fexthunks_exports_##name(void *a0, uintptr_t a1) { call_guest = (fex_call_callback_t*)a0; (uintptr_t&)callback_unpacks = a1; fexldr_init_##name(); return exports; } }
 
 #define LOAD_LIB_INIT(init_fn) \