openmp: Add support for the omp_get_supported_active_levels runtime library routine

This patch implements the omp_get_supported_active_levels runtime routine
from the OpenMP 5.0 specification, which returns the maximum number of
active nested parallel regions supported by this implementation.  The
current maximum (set using the omp_set_max_active_levels routine or the
OMP_MAX_ACTIVE_LEVELS environment variable) cannot exceed this number.

2020-10-13  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* env.c (gomp_max_active_levels_var): Initialize to
	gomp_supported_active_levels.
	(initialize_env): Limit gomp_max_active_levels_var to be at most
	equal to gomp_supported_active_levels.
	* fortran.c (omp_get_supported_active_levels): Add ialias_redirect.
	(omp_get_supported_active_levels_): New.
	* icv.c (omp_set_max_active_levels): Limit gomp_max_active_levels_var
	to at most equal to gomp_supported_active_levels.
	(omp_get_supported_active_levels): New.
	* libgomp.h (gomp_supported_active_levels): New.
	* libgomp.map (OMP_5.0.1): Add omp_get_supported_active_levels and
	omp_get_supported_active_levels_.
	* libgomp.texi (omp_get_supported_active_levels): New.
	(omp_set_max_active_levels): Update.  Add reference to
	omp_get_supported_active_levels.
	* omp.h.in (omp_get_supported_active_levels): New.
	* omp_lib.f90.in (omp_get_supported_active_levels): New.
	* omp_lib.h.in (omp_get_supported_active_levels): New.
	* testsuite/libgomp.c/lib-2.c (main): Check omp_get_max_active_levels
	against omp_get_supported_active_levels.
	* testsuite/libgomp.fortran/lib4.f90 (lib4): Likewise.