um: add __weak for exported functions

If the exported glibc functions don't exist, we get link
failures. Avoid that by adding __weak so they're allowed
to not exist.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/user_syms.c

@@ -39,7 +39,7 @@ EXPORT_SYMBOL(printf);
  * good; so the versions of these symbols will always match
  */
 #define EXPORT_SYMBOL_PROTO(sym)       \
-	int sym(void);                  \
+	int sym(void) __weak; \
 	EXPORT_SYMBOL(sym);
 
 extern void readdir64(void) __attribute__((weak));