Unthreaded RTS incorrectly attributes time to safe foreign calls

Currently the following program reports CPU time being spent in the MAIN cost centre when run using the non-threaded runtime. This is true despite the program being blocked in a safe foreign call:

{-# LANGUAGE ForeignFunctionInterface #-}

foreign import ccall safe "sleep" sleep :: Int -> IO ()

main = sleep 42

This disagrees with both the specification of the profiler's behavior and the threaded runtime.