build: Simplify piegen usage

- no need for phony pie/piegen, redundant and rather may
  cause double execution of a target;
- drop $(ARCH_DIR) from pie deps, initially it should be syscall-lib,
  but pie/lib.a already has it.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Tested-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>