Fix Mac build when assertion is disabled.

Change-Id: I8a99fea577473427cc4339ea53c1f5d1c7cd328d
diff --git a/lib/Target/X86/X86SectLinker.cpp b/lib/Target/X86/X86SectLinker.cpp
index a25a4c3..300b112 100644
--- a/lib/Target/X86/X86SectLinker.cpp
+++ b/lib/Target/X86/X86SectLinker.cpp
@@ -11,6 +11,7 @@
 
 #include "X86.h"
 #include "X86ELFSectLinker.h"
+#include <stdlib.h>
 
 using namespace mcld;
 
@@ -34,6 +35,8 @@
     return new X86ELFSectLinker(pOption, pLDBackend);
 
   assert(0 && "X86_64 has not supported yet");
+  abort();
+  return NULL;
 }
 
 } // namespace of mcld
@@ -44,4 +47,3 @@
   // Register the linker frontend
   mcld::TargetRegistry::RegisterSectLinker(TheX86Target, createX86SectLinker);
 }
-