Enable nanosecond-precision for VMThread.wait0

* README (JCGO_CLOCKGETTM): Update information.
* include/jcgothrd.h (JCGO_EVENTTIME_PREPARE): Add "ns" argument;
use clock_gettime() if JCGO_CLOCKGETTM but not JCGO_MONOTWAIT.
* include/jcgovthr.c (java_lang_VMThread__wait0__LoJI): Make "ns"
value zero if "ms" value exceeds the limit; pass "ns" value to
JCGO_EVENTTIME_PREPARE (if both "ms" and "ns" are zero then infinite
wait is performed).