[Hurd] Fix -Wswitch in Hurd::getDynamicLinker (NFC)

llvm-project/clang/lib/Driver/ToolChains/Hurd.cpp:137:11:
 error: 60 enumeration values not handled in switch: 'UnknownArch', 'arm', 'armeb'... [-Werror,-Wswitch]
  switch (getArch()) {
          ^~~~~~~~~
1 error generated.