We will need to use these to setup proper unwinding information for the
stg_stop_thread closure. This pokes a hole in the STG abstraction,
exposing the machine's stack pointer register so that we can accomplish
this. We also expose a dummy return address register, which corresponds
to the register used to hold the DWARF return address.

Differential Revision: https://phabricator.haskell.org/D1225