libstdc++: Define __cpp_lib_execution feature test macro

libstdc++: Define __cpp_lib_execution feature test macro

This macro has never been defined by libstdc++, despite supporting the
parallel algorithms. It should have a different value for C++17 and
C++20, because P1001R2 should not be supported in C++17, but
unsequenced_policy is defined for C++17 (see PR p4702).

	* include/std/execution (__cpp_lib_execution): Define to indicate
	support for P0024R2 and P1001R2.
	* include/std/version (__cpp_lib_execution): Define.
	* testsuite/25_algorithms/pstl/feature_test.cc: Only test macro
	defined by <algorithm>, move other tests to new tests ...
	* testsuite/25_algorithms/pstl/feature_test-2.cc: New test.
	* testsuite/25_algorithms/pstl/feature_test-3.cc: New test.
	* testsuite/25_algorithms/pstl/feature_test-4.cc: New test.
	* testsuite/25_algorithms/pstl/feature_test-5.cc: New test.