stg_gc_unpt_r1_ret: if R1 isn't mapped to a real register,
keep it on the stack.

I confess to 'pushing symbols' here a bit, as the exact
nature (and function) of stg_gc_unpt_r1_info isn't clear
to me (documentation isn't abundant, for one.) However,
it will now behave in a manner consistent with what
the code generator emits in the unregisterised case.