Skip 30_threads/future/members/poll.cc on hppa*-*-linux*

hppa*-*-linux* lacks high resolution timer support. Timer resolution
ranges from 1 to 10ms. As a result, a large number of iterations are
needed for the wait_for_0 and ready loops. This causes the
wait_until_sys_epoch and wait_until_steady_epoch loops to timeout.
There the loop wait time is determined by the timer resolution.

2024-07-04  John David Anglin  <danglin@gcc.gnu.org>

libstdc++-v3/ChangeLog:
	PR libstdc++/98678
	* testsuite/30_threads/future/members/poll.cc: Skip on hppa*-*-linux*.