libstdc++: Fix std::counting_semaphore<> default max value

libstdc++: Fix std::counting_semaphore<> default max value

My recent (uncommitted) changes to support a 64-bit __platform_wait_t
for FreeBSD and Darwin revealed a problem in std::counting_semaphore.
When the default template argument is used and __platform_wait_t is a
64-bit type, the numeric_limits<__platform_wait_t>::max() value doesn't
fit in ptrdiff_t and so we get ptrdiff_t(-1), which fails a
static_assert in the class body.

The solution is to cap the value to PTRDIFF_MAX instead of allowing it
to go negative.

libstdc++-v3/ChangeLog:

	* include/bits/semaphore_base.h (__platform_semaphore::_S_max):
	Limit to PTRDIFF_MAX to avoid negative values.
	* testsuite/30_threads/semaphore/least_max_value.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>