time: Fix compile error in itimer test affecting hurd

The recent change to use __KERNEL_OLD_TIMEVAL_MATCHES_TIMEVAL64 to avoid
doing 64-bit checks on some platforms broke the test for hurd where
__KERNEL_OLD_TIMEVAL_MATCHES_TIMEVAL64 is not defined.  With error:

    tst-itimer.c: In function 'do_test':
    tst-itimer.c:103:11: error: '__KERNEL_OLD_TIMEVAL_MATCHES_TIMEVAL64' undeclared (first use in this function)
      103 |       if (__KERNEL_OLD_TIMEVAL_MATCHES_TIMEVAL64)
	  |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tst-itimer.c:103:11: note: each undeclared identifier is reported only once for each function it appears in

Define a support helper to detect when setitimer and getitimer support
64-bit time_t.

Fixes commit 6e8a0aac ("time: Fix overflow itimer tests on 32-bit
systems").

Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: Joseph Myers <joseph@codesourcery.com>