s390: Fix Enable VDSO for static linking

The commit 5e855c89
"s390: Enable VDSO for static linking" removed the definition of VDSO_SETUP
which leads to not setup the vdso symbols.
Instead it jumps to false addresses.

This patch just re adds the removed VDSO_SETUP macro definition.

ChangeLog:

	* sysdeps/unix/sysv/linux/s390/init-first.c (VDSO_SETUP): New define.