RFR 8191872: (fs) UnixNativeDispatcher conditionally compiles in support for high precision timestamps

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8191872
http://cr.openjdk.java.net/~bpb/8191872/webrev.00/

1. Add conditional compilation branch for macOS to support higher precision so Files.getLastModifiedTime() has the same milliseconds precision as File.lastModified() which was already updated in JDK 10 [1].
2. Remove conditional compilation restriction on non-macOS Unix, i.e., (_POSIX_C_SOURCE >= 200809L) || defined(solaris).

Thanks,

Brian

[1] https://bugs.openjdk.java.net/browse/JDK-8177809