Return has_DT_SYMBOLIC flag.

This reverts commit 8f61d99183

 Despite the fact that static linker does all the work while linking
 -Bsymbolic executables, according to the SCO doc following DT_SYMBOLIC
 and DF_SYMBOLIC flags is still a requirement for the dynamic linker
 as well.

 (see http://www.sco.com/developers/gabi/2012-12-31/ch5.dynamic.html)

Change-Id: I23f6a5e9b662e40cdbc462b64feff912431e800c

linker/linker.h

@@ -197,7 +197,7 @@ struct soinfo {
 #if !defined(__LP64__)
   bool has_text_relocations;
 #endif
-  bool unused4; // DO NOT USE, maintained for compatibility
+  bool has_DT_SYMBOLIC;
 
   soinfo(const char* name, const struct stat* file_stat, int rtld_flags);