libstdc++: Refactor loops in std::__platform_semaphore

libstdc++: Refactor loops in std::__platform_semaphore

Refactor the loops to all use the same form, and to not need explicit
'break' or 'continue' jumps. This also avoids a -Wunused-variable
warning with -Wsystem-headers.

Also fix a bug for absolute timeouts specified with a time that isn't
implicitly convertible to __clock_t::time_point, e.g. one with a higher
resolution such as picoseconds. Use chrono::ceil to round up to the next
time point representable by the clock.

libstdc++-v3/ChangeLog:

	* include/bits/semaphore_base.h (__platform_semaphore): Refactor
	loops to all use similar forms.
	(__platform_semaphore::_M_try_acquire_until): Use chrono::ceil
	to explicitly convert to __clock_t::time_point.
	* testsuite/30_threads/semaphore/try_acquire_for.cc: Check that
	using a very high resolution timeout compiles.
	* testsuite/30_threads/semaphore/platform_try_acquire_for.cc:
	New test.