[MachOWriter] Initialize the values for PtrAuth fields

Initialize some fields in MachOObjectWriter that was moved from
ObjectWriter. Fix the UBSAN tests failed for uninitialized variables.

(cherry picked from commit 76bb8e2)

Author: cachemeifyoucan

llvm/include/llvm/MC/MCMachObjectWriter.h

@@ -154,8 +154,8 @@ class MachObjectWriter : public MCObjectWriter {
   VersionInfoType VersionInfo{};
   VersionInfoType TargetVariantVersionInfo{};
 
-  std::optional<unsigned> PtrAuthABIVersion;
-  bool PtrAuthKernelABIVersion;
+  std::optional<unsigned> PtrAuthABIVersion = std::nullopt;
+  bool PtrAuthKernelABIVersion = false;
 
   MachSymbolData *findSymbolData(const MCSymbol &Sym);