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.
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