x86/vdso/doc: Make vDSO examples more portable

This adds a new vdso_test.c that's written entirely in C.  It also
makes all of the vDSO examples work on 32-bit x86.

Cc: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/62b701fc44b79f118ac2b2d64d19965fc5c291fb.1402620737.git.luto@amacapital.net


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