Revert "[ELF] Relax TLSLD error check"

This reverts commit 1baac27.

In order to link gcc 7's output, relaxing this error check wasn't
enough as reported on #145.
We need to remove more error checks or keep them as-is.
For now, I'll restore all the error checks, but before making another
release, I may remove all error checks to accept gcc 7's buggy
output files.

elf/arch-x86-64.cc

@@ -629,15 +629,9 @@ void InputSection<X86_64>::scan_relocations(Context<X86_64> &ctx) {
     case R_X86_64_TLSLD:
       if (i + 1 == rels.size())
         Fatal(ctx) << *this
-                   << ": TLSLD reloc must be followed by PLT32 or GOTPCREL";
-
-      if (sym.is_imported) {
-        // This should be a fatal error as well, but it looks like GCC 7
-        // wrongly creates such relocations against STB_GNU_UNIQUE symbols.
-        // Other linkers don't have this error check. So, for bug-compatibility,
-        // we only warn about it and create a (possibly corrupted) output file.
-        Warn(ctx) << *this << ": TLSLD reloc refers external symbol: " << sym;
-      }
+                   << ": TLSGD reloc must be followed by PLT32 or GOTPCREL";
+      if (sym.is_imported)
+        Fatal(ctx) << *this << ": TLSLD reloc refers external symbol: " << sym;
 
       if (ctx.arg.relax && !ctx.arg.shared)
         i++;