Generate __data_start symbol that marks the start of .data when __data_start
is referenced from code being linked.

Index: tools/lld/ELF/Symbols.h
--- tools/lld/ELF/Symbols.h.orig
+++ tools/lld/ELF/Symbols.h
@@ -436,6 +436,9 @@ struct ElfSym {
   // __bss_start
   static Defined *bss;
 
+  // __data_start
+  static Defined *data;
+
   // etext and _etext
   static Defined *etext1;
   static Defined *etext2;
