Add CLOCK_REALTIME_FAST for FreeBSD.

I got the value from here:

http://fxr.watson.org/fxr/source/sys/time.h

This is required for Phobos pull# 2584.
I know it's super ugly to have circular dependencies, but declaring a symbol here and there causes conflicts, because a common usage looks like this.
```d
import core.sys.posix.time; // defines deprecated CLOCK_MONOTONIC_RAW
//...
version (linux) {
    import core.sys.linux.time; // also defines CLOCK_MONOTONIC_RAW
    clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
} else version (Posix) {
    clock_gettime(CLOCK_MONOTONIC, &ts);
}
```
I went with deprecated aliases to make this work.
https://github.com/D-Programming-Language/druntime/pull/982/files#diff-15dcf0d00249644acc3bf50572270cb4R254