[Object][COFF][NFC] Introduce getMachineArchType helper. (#87370)

It's a common pattern that we have a machine type, but we don't care
which ARM64* platform we're dealing with. We already have
isAnyArm64 for that, but it does not fit cases where we use a switch
statement. With this helper, it's easy to simplify such cases by using
Triple::ArchType instead of machine type.