libstdc++: Use std::conditional_t instead of lambda to select semaphore implementation

libstdc++: Use std::conditional_t instead of lambda to select semaphore implementation

The lambda expression causes testsuite failures such as:
FAIL g++.dg/modules/xtreme-header-2_b.C -std=c++26 (test for excess errors)

libstdc++-v3/ChangeLog:

	* include/bits/semaphore_base.h (_Select_semaphore_impl): Rename
	to _Semaphore_impl and use std::conditional_t instead of an
	immediately invoked lambda expression.
	* include/std/semaphore (counting_semaphore): Adjust to use new
	name.