Y2038: make __tz_convert compatible with 64-bit-time

Now that __time64_t exists, we can switch internal function
__tz_convert from 32-bit to 64-bit time. This involves switching
some other internal functions as well, namely __tz_compute and
__offtime.

Tested with 'make check' on x86_64-linux-gnu and i686-linux.gnu.

	* include/time.h
	(__tz_compute): Replace time_t with __time64_t.
	(__tz_convert): Replace time_t* with __time64_t.
	(__offtime): Replace time_t* with __time64_t.
	* time/gmtime.c
	(__gmtime_r): Adjust call to __tz_convert.
	(gmtime): Likewise.
	* time/localtime.c
	(__localtime_r): Likewise.
	(localtime): Likewise.
	* time/offtime.c: Replace time_t with __time64_t.
	* time/tzset.c: Likewise.