[gn build] Port 87104fa

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lld/MachO/BUILD.gn

@@ -22,6 +22,7 @@ static_library("MachO2") {
     "//libunwind/include",
   ]
   sources = [
+    "Arch/ARM64.cpp",
     "Arch/X86_64.cpp",
     "Driver.cpp",
     "DriverUtils.cpp",