lld: fix crash linking the FreeBSD/arm64 kernel

As reported by dim@ in LLVM PR33821 lld was crashing while linking the
FreeBSD/arm64 kernel, with a null linker script command section:

  1337      Cmd->Sec->addSection(Sentinel);
  (gdb) p Cmd->Sec
  $4 = (lld::elf::OutputSection *) 0x0

Under discussion upstream, https://llvm.org/pr33821.  Not yet certain if
this fix is correct, but it does prevent the crash and the LLD test
suite passes with the change.

contrib/llvm/tools/lld/ELF/Writer.cpp

@@ -1331,7 +1331,7 @@ template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
   // ARM ABI requires .ARM.exidx to be terminated by some piece of data.
   // We have the terminater synthetic section class. Add that at the end.
   OutputSectionCommand *Cmd = findSectionCommand(".ARM.exidx");
-  if (!Cmd || Cmd->Commands.empty() || Config->Relocatable)
+  if (!Cmd || !Cmd->Sec || Cmd->Commands.empty() || Config->Relocatable)
     return;
 
   auto *Sentinel = make<ARMExidxSentinelSection>();