libsupport: Add 64-bit time_t support for time functions

The patch adds redirections for xclock_gettime, xclock_settime,
timespec_add, timespec_sub, test_timespec_before_impl,
test_timespec_equal_or_after_impl, support_timespec_ns,
support_timespec_normalize, and support_timespec_check_in_range when
_TIME_BITS=64 is defined.

Co-authored-by: Lukasz Majewski <lukma@denx.de>
Tested-by: Carlos O'Donell <carlos@redhat.com>