libstdc++: Remove redundant clock conversions in atomic waits

For the case where a timeout is specified using the system_clock we
perform a conversion to the preferred clock (which is either
steady_clock or system_clock itself), wait using __cond_wait_until_impl,
and then check the time by that clock again to see if it was reached.
This is entirely redundant, as we can just call __cond_wait_until_impl
directly. It will wait using the specified clock, and there's no need to
check the time twice. For the no_timeout case this removes two
unnecessary calls to the clock's now() function, and for the timeout
case it removes three calls.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__cond_wait_until): Do not
	perform redundant conversions to the same clock.

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