[lldb] Recognize MachO cpu sub-type 2 as arm64

[JDevlieghere]

Summary

Make sure we recognize cpu sub-type 2 as arm64. In reality it's arm64e,
but we don't have the triple for that. Without this patch, we fall back
to unknown-apple-macosx- for the default architecture, which breaks
things like running expressions without a target.

[jasonmolenda]

Bien.

lldb/source/Utility/ArchSpec.cpp

static const ArchDefinitionEntry g_macho_arch_entries[] = {
     {ArchSpec::eCore_arm_armv7s, llvm::MachO::CPU_TYPE_ARM, 11, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_armv7k, llvm::MachO::CPU_TYPE_ARM, 12, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_armv7m, llvm::MachO::CPU_TYPE_ARM, 15, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_armv7em, llvm::MachO::CPU_TYPE_ARM, 16, UINT32_MAX,
      SUBTYPE_MASK},
+    {ArchSpec::eCore_arm_arm64, llvm::MachO::CPU_TYPE_ARM64, 2, UINT32_MAX,
+     SUBTYPE_MASK}, // FIXME: This should be arm64e once the triple exists.
     {ArchSpec::eCore_arm_arm64, llvm::MachO::CPU_TYPE_ARM64, 1, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_arm64, llvm::MachO::CPU_TYPE_ARM64, 0, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_arm64, llvm::MachO::CPU_TYPE_ARM64, 13, UINT32_MAX,
      SUBTYPE_MASK},
     {ArchSpec::eCore_arm_arm64_32, llvm::MachO::CPU_TYPE_ARM64_32, 0,
      UINT32_MAX, SUBTYPE_MASK},