s390/vdso: check for undefined symbols after build

When adding an undefined symbol the build still succeeds, but
userspace is crashing trying to execute vdso because the
undefined symbol is not resolved. Add the check for undefined
symbols to prevent this.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>