GHCi: use non-updatable thunks for breakpoints

GHCi: use non-updatable thunks for breakpoints
The extra safe points introduced for breakpoints were previously
compiled as normal updatable thunks, but they are guaranteed
single-entry, so we can use non-updatable thunks here.  This restores
the tail-call property where it was lost in some cases (although stack
squeezing probably often recovered it), and should improve
performance.