Support parallel testing in libgomp, part I [PR66005]

..., while still hard-coding the number of parallel slots to one.

	PR testsuite/66005
	libgomp/
	* testsuite/Makefile.am (PWD_COMMAND): New variable.
	(%/site.exp): New target.
	(check_p_numbers0, check_p_numbers1, check_p_numbers2)
	(check_p_numbers3, check_p_numbers4, check_p_numbers5)
	(check_p_numbers6, check_p_numbers, gcc_test_parallel_slots)
	(check_p_subdirs)
	(check_DEJAGNU_libgomp_targets): New variables.
	($(check_DEJAGNU_libgomp_targets)): New target.
	($(check_DEJAGNU_libgomp_targets)): New dependency.
	(check-DEJAGNU $(check_DEJAGNU_libgomp_targets)): New targets.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/lib/libgomp.exp: For parallel testing,
	'load_file ../libgomp-test-support.exp'.

Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>