x86: ifdef 32-bit specific setup in init_memory_mapping()

Impact: cleanup

Enabling NX, PSE, and PGE are only required on 32-bit so ifdef them
in both versions of the function.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-5-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>