kselftest: Extend vDSO selftest

The current version of the multiarch vDSO selftest verifies only
gettimeofday.

Extend the vDSO selftest to the other library functions:
 - time
 - clock_getres
 - clock_gettime

The extension has been used to verify the unified vdso library on the
supported architectures.

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>