y2038: Provide conversion helpers for struct __ntptimeval64

Those functions allow easy conversion between Y2038 safe, glibc internal
struct __ntptimeval64 and struct ntptimeval.

The reserved fields (i.e. __glibc_reserved{1234}) during conversion are
zeroed as well, to provide behavior similar to one in ntp_gettimex function
(where those are cleared before the struct ntptimeval is returned).

Those functions are put in Linux specific sys/timex.h file, as putting
them into glibc's local include/time.h would cause build break on HURD as
it doesn't support struct timex related syscalls.

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>