BUILD: tools: do not use the weak attribute for trace() on obsolete linkers

The default dummy trace() function is marked weak in order to be easily
replaced at link time. Some linkers are having issues with the weak
attribute, so let's not mark it on these linkers. They will simply not
be able to build with TRACE=1, which is no big deal since it's only used
by developers.

src/standard.c

@@ -4298,7 +4298,9 @@ int parse_dotted_uints(const char *str, unsigned int **nums, size_t *sz)
 }
 
 /* do nothing, just a placeholder for debugging calls, the real one is in trace.c */
+#ifndef USE_OBSOLETE_LINKER
 __attribute__((weak,format(printf, 1, 2)))
+#endif
 void trace(char *msg, ...)
 {
 }