hi/low addr space bits can be sent in stop-rely packet

Add support for the `low_mem_addressing_bits` and
`high_mem_addressing_bits` keys in the stop reply packet,
in addition to the existing `addressing_bits`.  Same
behavior as in the qHostInfo packet.

Clean up AddressableBits so we don't need to check if
any values have been set in the object before using it
to potentially update the Process address masks.

Differential Revision: https://reviews.llvm.org/D158041