a68: Add exit function to POSIX prelude

a68: Add exit function to POSIX prelude

Add the procedure `posixexit'.

gcc/algol68/ChangeLog:

	* a68-low-posix.cc (a68_posix_setexitstatus): Delete function.
	(a68_posix_exit): New function.
	* a68-low-prelude.cc (a68_lower_setexitstatus): Delete function.
	(a68_lower_posixexit): New function.
	* a68-low-runtime.def (SET_EXIT_STATUS): Delete definition.
	(POSIX_EXIT): Add definition for posixexit.
	* a68-parser-prelude.cc (posix_prelude): Remove setexitstatus
	identifier from and add posixexit identifier to standenv.
	* a68.h (a68_posix_setexitstatus): Delete prototype.
	(a68_lower_setexitstatus): Likewise.
	(a68_posix_exit): New prototype.
	(a68_lower_posixexit): Likewise.
	* ga68.texi:

libga68/ChangeLog:

	* ga68-posix.c (_libga68_posixexit): New function.
	* ga68.h (_libga68_posixexit): New prototype.
	(_libga68_set_exit_status): Delete prototype.
	* ga68.map: Remove _libga68_set_exit_status from and add
	_libga68_posixexit to the global map.
	* libga68.c: include <stdlib.h>.
	(_libga68_set_exit_status): Delete function.
	(main): Return EXIT_SUCCESS.

gcc/testsuite/ChangeLog:

	* algol68/execute/posix-exit-1.a68: New test.

Signed-off-by: Pietro Monteiro <pietro@sociotechnical.xyz>