libgomp: Add tests for omp_atv_serialized and deprecate omp_atv_sequential.

The variable omp_atv_sequential was replaced by omp_atv_serialized in OpenMP
5.1. This was already implemented by Jakub (C/C++, commit ea82325a) and
Tobias (Fortran, commit fff15bad).

This patch adds two tests to check if omp_atv_serialized is available (one test
for C/C++ and one for Fortran). Besides that omp_atv_sequential is marked as
deprecated in C/C++ and Fortran for OpenMP 5.1.

libgomp/ChangeLog:

	* allocator.c (omp_init_allocator): Replace omp_atv_sequential with
	omp_atv_serialized.
	* omp.h.in: Add deprecated flag for omp_atv_sequential.
	* omp_lib.f90.in: Add deprecated flag for omp_atv_sequential.
	* testsuite/libgomp.c-c++-common/alloc-10.c: New test.
	* testsuite/libgomp.fortran/alloc-12.f90: New test.