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 <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: H. Peter Anvin <[e-mail]>

Documentation/vDSO/vdso_test.c Documentation/vDSO/vdso_standalone_test_x86.c

···
  * You can amuse yourself by compiling with:
  * gcc -std=gnu99 -nostdlib
  *     -Os -fno-asynchronous-unwind-tables -flto
- *      vdso_test.c parse_vdso.c -o vdso_test
+ *      vdso_standalone_test_x86.c parse_vdso.c
  * to generate a small binary with no dependencies at all.
  */