vdso: share the vDSO proxy initialization between all architectures

This patch splits the file arch/x86/vdso.c into machine-independent
and machine-dependent parts by moving the routine vdso_init()
from the file vdso.c. The routine seems to be suitable for all
architectures supporting the vDSO.

The ARM version of the routine is moved to the source vdso-stub.c
that is supposed to be the vDSO proxy stub implementation for
architectures that don't provide the vDSO. The build scripts are
adjusted as well to enable selection between the full-fledged
and stub vDSO proxy implementations.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Looks-good-to: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>