Drop platform specific versions of gethrtime()

Use the implementation from C++ unless the platform supports
a native one.

Change-Id: I480bbd8953ce6f3cfe27e9e0f4b8c9f6b30385ff
Reviewed-on: http://review.couchbase.org/44078
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>