selftests/x86: Add a missing .note.GNU-stack section to thunks_32.S

test_syscall_vdso_32 ended up with an executable stacks because the asm
was missing the annotation that says that it is modern and doesn't need
an executable stack. Add the annotation.

This was missed in commit aeaaf005 ("selftests/x86: Add missing
.note.GNU-stack sections").

Fixes: aeaaf005 ("selftests/x86: Add missing .note.GNU-stack sections")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/487ed5348a43c031b816fa7e9efedb75dc324299.1614877299.git.luto@kernel.org