Remove a special handling of AMDGPU entry points.

[ruiu]

Summary

This is the last peculiar semantics left in the linker, and I'd
like to remove it to streamline it. If you want to always set an
entry point to 0, you can pass -e 0 to the linker. I think
doing it explicitly is much better rather than having special
code for AMDGPU in the linker.

[• tstellarAMD]

LGTM.

ELF/Driver.cpp

 // Some command line options or some combinations of them are not allowed.
 // This function checks for such errors.
 static void checkOptions(opt::InputArgList &Args) {
   // The MIPS ABI as of 2016 does not support the GNU-style symbol lookup
   // table which is a relatively new feature.
   if (Config->EMachine == EM_MIPS && Config->GnuHash)
     error("the .gnu.hash section is not compatible with the MIPS target.");
 
-  if (Config->EMachine == EM_AMDGPU && !Config->Entry.empty())
-    error("-e option is not valid for AMDGPU.");
-
   if (Config->Pie && Config->Shared)
     error("-shared and -pie may not be used together");
 
   if (Config->Relocatable) {
     if (Config->Shared)
       error("-r and -shared may not be used together");
     if (Config->GcSections)
       error("-r and --gc-sections may not be used together");
template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   Config->Mips64EL =
       (Config->EMachine == EM_MIPS && Config->EKind == ELF64LEKind);
   Config->ImageBase = getImageBase(Args);
 
   // Default output filename is "a.out" by the Unix tradition.
   if (Config->OutputFile.empty())
     Config->OutputFile = "a.out";
 
-  // Use default entry point name if -e was missing. AMDGPU binaries
-  // have no entries. For some reason, MIPS' entry point name is
-  // different from others.
-  if (Config->Entry.empty() && !Config->Relocatable &&
-      Config->EMachine != EM_AMDGPU)
+  // Use default entry point name if -e was missing.
+  // For some reason, MIPS' entry point name is different from others.
+  if (Config->Entry.empty() && !Config->Relocatable)
     Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
 
   // Handle --trace-symbol.
   for (auto *Arg : Args.filtered(OPT_trace_symbol))
     Symtab.trace(Arg->getValue());
 
   // Initialize Config->MaxPageSize. The default value is defined by
   // each target.