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

Index: tools/lld/ELF/Symbols.cpp
--- tools/lld/ELF/Symbols.cpp.orig
+++ tools/lld/ELF/Symbols.cpp
@@ -48,6 +48,7 @@ std::string lld::toELFString(const Archive::Symbol &b)
 }
 
 Defined *ElfSym::bss;
+Defined *ElfSym::data;
 Defined *ElfSym::etext1;
 Defined *ElfSym::etext2;
 Defined *ElfSym::edata1;