Fix a scheduling/GC bug, spotted by Wolfgang Thaller.  If a main
thread completes, and a GC runs before the return (from rts_evalIO())
happens, then the thread might be GC'd before we get a chance to
extract its return value, leading to barf("main thread has been GC'd")
from the garbage collector.

The fix is to treat all main threads which have completed as roots:
this is logically the right thing to do, because these threads must be
retained by virtue of holding the return value, and this is a property of
main threads only.