selftests: vdso_test: support ARM64 targets

ARM64's vDSO exports its gettimeofday() implementation with a different
name (__kernel_gettimeofday) and version (LINUX_2.6.39) from other
architectures.  Add a corresponding special-case to vdso_test.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>