Improve the handling of threadDelay in the non-threaded RTS

Firstly, we were rounding up too much, such that the smallest delay
was 20ms.  Secondly, there is no need to use millisecond resolution on
a 64-bit machine where we have room in the TSO to use the normal
nanosecond resolution that we use elsewhere in the RTS.

MERGED from commit 9d26519c