clock_adjtime: generalize time64 not to assume old struct layout match

clock_adjtime: generalize time64 not to assume old struct layout match

commit 2b4fd6f75b4fa66d28cddcf165ad48e8fda486d1 added time64 for this
function, but did so with a hidden assumption that the new time64
version of struct timex will be layout-compatible with the old one.
however, there is little benefit to doing it that way, and the cost is
permanent special-casing of 32-bit archs with 64-bit time_t in the
public interface definitions.

instead, do a full translation of the structure going in and out. this
commit is actually a revision to an earlier uncommited version of the
code.