[BOLT][DWARF][NFC] Add mc opt to DWARFRewriter.cpp (llvm#100800)

Running into an error with removing DWP where the assertion
`RelaxAllView &&
"RegisterMCTargetOptionsFlags not created."'` failed. This is a result
of DWP bringing the mc::RegisterMCTargetOptionsFlags option in, and the
option being removed with DWP. The need for this option didn't
originally exist because we didn't use MC in DWARFRewriter, but we
switched to using DWARFStreamer which needed the option.

https://reviews.llvm.org/D75579 
https://reviews.llvm.org/D106417

bolt/lib/Rewrite/DWARFRewriter.cpp

@@ -32,6 +32,7 @@
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCStreamer.h"
+#include "llvm/MC/MCTargetOptionsCommandFlags.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
@@ -56,6 +57,8 @@
 #undef DEBUG_TYPE
 #define DEBUG_TYPE "bolt"
 
+static mc::RegisterMCTargetOptionsFlags MOF;
+
 static void printDie(const DWARFDie &DIE) {
   DIDumpOptions DumpOpts;
   DumpOpts.ShowForm = true;