selftests/x86: Add missing .note.GNU-stack sections

Several of the x86 selftests end up with executable stacks because
the asm was missing the annotation that says that they are modern
and don't need executable stacks.  Add the annotations.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/6f043c03e9e0e4557e1e975a63b07a4d18965a68.1604346596.git.luto@kernel.org