x86/CET: Add a re-exec test with legacy bitmap

Add a re-exec test with legacy bitmap to verify that legacy bitmap is
properly hanlded by kernel.

	* sysdeps/x86/Makefile (tests): Add tst-cet-legacy-1a.
	(tst-cet-legacy-1a-ARGS): New.
	($(objpfx)tst-cet-legacy-1a): New target.
	* sysdeps/x86/tst-cet-legacy-1a.c: New file.