Ensure __executable_start is added to executables

[thomasanderson]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=40134

[thomasanderson]

+pcc
no idea if this is the right fix

[pcc]

This would seem to add the reserved symbols to object file outputs (i.e. ld -r), which isn't correct.

[thomasanderson]

Turns out this patch doesn't even fix the bug, but I've debugged it and I think I know what the issue is now. I'll send out a better patch soon

ELF/Driver.cpp

if (Args.hasArg(OPT_exclude_libs))
     excludeLibs<ELFT>(Args);
 
   // Create ElfHeader early. We need a dummy section in
   // addReservedSymbols to mark the created symbols as not absolute.
   Out::ElfHeader = make<OutputSection>("", 0, SHF_ALLOC);
   Out::ElfHeader->Size = sizeof(typename ELFT::Ehdr);
 
   // We need to create some reserved symbols such as _end. Create them.
-  if (!Config->Relocatable)
-    addReservedSymbols();
 
   // Apply version scripts.
   //
   // For a relocatable output, version scripts don't make sense, and
   // parsing a symbol version string (e.g. dropping "@ver1" from a symbol
   // name "foo@ver1") rather do harm, so we don't call this if -r is given.
   if (!Config->Relocatable)
     Symtab->scanVersionScript();