We need this, or something equivalent, to be able to implement
stgAllocForGMP outside of the rts. That's because we want to use
allocateLocal which allocates from the given capability without
having to take any locks. In the gmp primops we're basically in
an unsafe foreign call, that is a context where we hold a current
capability. So it's safe for us to use allocateLocal. We just
need a way to get the current capability. The method to get the
current capability varies depends on whether we're using the
threaded rts or not. When stgAllocForGMP is built inside the rts
that's ok because we can do it conditionally on THREADED_RTS.
Outside the rts we need a single api we can call without knowing
if we're talking to a threaded rts or not, hence this addition.