Documentation: disable vdso_test to avoid breakage with old glibc

glibc versions older than 2.16 don't include sys/auxv.h which this
executable uses.
Since we don't have a good way to test for specific glibc versions in
kbuild, just disable it for now.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>