[lld][WebAssembly] Fix data section name in PIC mode

[sbc100]

Summary

This should always have been ".data". Without this we treat the
section as a user-defined section in other places (such as the
generation of __start/__stop symbols).

[dschuff]

LGTM

lld/wasm/Writer.cpp

 
 // If a section name is valid as a C identifier (which is rare because of
 // the leading '.'), linkers are expected to define __start_<secname> and
 // __stop_<secname> symbols. They are at beginning and end of the section,
 // respectively. This is not requested by the ELF standard, but GNU ld and
 // gold provide the feature, and used by many programs.
 static void addStartStopSymbols(const OutputSegment *Seg) {
   StringRef Name = Seg->Name;
-  LLVM_DEBUG(dbgs() << "addStartStopSymbols: " << Name << "\n");
   if (!isValidCIdentifier(Name))
     return;
+  LLVM_DEBUG(dbgs() << "addStartStopSymbols: " << Name << "\n");
   uint32_t Start = Seg->StartVA;
   uint32_t Stop = Start + Seg->Size;
   Symtab->addOptionalDataSymbol(Saver.save("__start_" + Name), Start);
   Symtab->addOptionalDataSymbol(Saver.save("__stop_" + Name), Stop);
 }
 
 void Writer::addSections() {
   addSection(Out.DylinkSec);
for (InputEvent *Event : File->Events)
       Out.EventSec->addEvent(Event);
   }
 }
 
 static StringRef getOutputDataSegmentName(StringRef Name) {
   // With PIC code we currently only support a single data segment since
   // we only have a single __memory_base to use as our base address.
   if (Config->Pic)
-    return "data";
+    return ".data";
   if (!Config->MergeDataSegments)
     return Name;
   if (Name.startswith(".text."))
     return ".text";
   if (Name.startswith(".data."))
     return ".data";
   if (Name.startswith(".bss."))
     return ".bss";