vdso: merge per arch vdso.c file in a common one

Merge arch/*/vdso.c files in vdso.c.

The merge has been done by copying arch/x86/vdso.c in vdso.c since it
contains patch a67e9f7b ("vdso: don't play with a function exit code").

The commit f9ae6d9d ("Replace remaining hard-coded TASK_SIZE use")
pushed in the aarch64 has been replicated since it should be cross
platform.

CC: Christopher Covington <cov@codeaurora.org>
CC: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>