[#317] template.core: remove _EXFUN reference

- in GCC 7.3.1, newlib changed this macro, so better avoid it

Author: ilg-ul

bundles/ilg.gnumcueclipse.templates.core/templates/common/system/src/newlib/_syscalls.c

@@ -592,7 +592,7 @@ newslot (void);
 register char* stack_ptr asm ("sp");
 
 /* following is copied from libc/stdio/local.h to check std streams */
-extern void _EXFUN(__sinit,(struct _reent*));
+extern void __sinit(struct _reent*);
 #define CHECK_INIT(ptr) \
   do                                            \
     {                                           \