openmp: Retire nest-var ICV for OpenMP 5.1

This removes the nest-var ICV, expressing nesting in terms of the
max-active-levels-var ICV instead.  The max-active-levels-var ICV
is now per data environment rather than per device.

2020-11-18  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* env.c (gomp_global_icv): Remove nest_var field.  Add
	max_active_levels_var field.
	(gomp_max_active_levels_var): Remove.
	(parse_boolean): Return true on success.
	(handle_omp_display_env): Express OMP_NESTED in terms of
	max_active_levels_var.  Change format specifier for
	max_active_levels_var.
	(initialize_env): Set max_active_levels_var from
	OMP_MAX_ACTIVE_LEVELS, OMP_NESTED, OMP_NUM_THREADS and
	OMP_PROC_BIND.
	* icv.c (omp_set_nested): Express in terms of
	max_active_levels_var.
	(omp_get_nested): Likewise.
	(omp_set_max_active_levels): Use max_active_levels_var field instead
	of gomp_max_active_levels_var.
	(omp_get_max_active_levels): Likewise.
	* libgomp.h (struct gomp_task_icv): Remove nest_var field.  Add
	max_active_levels_var field.
	(gomp_supported_active_levels): Set to UCHAR_MAX.
	(gomp_max_active_levels_var): Delete.
	* libgomp.texi (omp_get_nested): Update documentation.
	(omp_set_nested): Likewise.
	(OMP_MAX_ACTIVE_LEVELS): Likewise.
	(OMP_NESTED): Likewise.
	(OMP_NUM_THREADS): Likewise.
	(OMP_PROC_BIND): Likewise.
	* parallel.c (gomp_resolve_num_threads): Replace reference
	to nest_var with max_active_levels_var.  Use max_active_levels_var
	field instead of gomp_max_active_levels_var.