libstdc++: testsuite: tolerate non-cancelling sleep

libstdc++: testsuite: tolerate non-cancelling sleep

Though sleep, nanosleep and clock_nanosleep are all POSIX cancellation
points, not all target systems follow this POSIX requirement.
30_threads/thread/native_handle/cancel.cc will run until it times out
on such systems.

Rather than failing a C++ library test because of a limitation of the
target system, this patch gives the test a chance to successfully
exercise the features it intends to exercise, by introducing a
cancellation point in a loop that would otherwise run indefinitely on
systems exhibiting this limitation.

for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/thread/native_handle/cancel.cc: Add an
	explicit cancellation point in case sleep_for lacks one.