Add CLOCK_REALTIME_FAST for FreeBSD.

Added the platform-specific CLOCK_ values for Linux and FreeBSD and put
them in their own files, deprecating the pre-existing ones in the posix
file.

This is required for Phobos pull# 2584.
Merge pull request #981 from jmdavis/CLOCK_REALTIME_FAST

Add CLOCK_REALTIME_FAST for FreeBSD.