x86/vdso/doc: Rename vdso_test.c to vdso_standalone_test_x86.c

This thing is hopelessly x86_64-specific: it's an example of how to
access the vDSO without any runtime support at all.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/3efc170e0e166e15f0150c9fdb37d52488b9c0a4.1402620737.git.luto@amacapital.net


Signed-off-by: H. Peter Anvin <hpa@zytor.com>