s390: Enable VDSO for static linking

Although s390 only enables vDSO for dynamically linked elf binaries
(arch/s390/kernel/vdso.c:217), there is no indication in the code or
associated commit message for why not enable it for statically linked
binaries as well.  To double check, I rebuilt a kernel with the
check removed and the vDSO does work for static build for supplied
symbols.

Checked on s390x-linux-gnu and s390-linux-gnu.

	[BZ #19767]
	* sysdeps/unix/sysv/linux/s390/init-first.c: Remove #ifdef SHARED.
	* sysdeps/unix/sysv/linux/s390/libc-vdso.h: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/sysdep.h
	(ALWAYS_USE_VSYSCALL): Define.
	* sysdeps/unix/sysv/linux/s390/s390-64/sysdep.h
	(ALWAYS_USE_VSYSCALL): Likewise.