`pte_flags` bilge conversion [draft]

[NathanRoyer]

Here is my partial bilge conversion of pte_flags.

At this point the only thing I converted was the PteFlagsAarch64 struct, I didn't update the methods from the bitflags era. As such, this doesn't compile, but the definition of that struct should be correct. When the field is defined as an enum, I moved the field documentation to the enum, not sure if that's a good thing to do.