x86: ifdef 32-bit and 64-bit setup in init_memory_mapping()

Impact: cleanup

To reduce the diff between the 32-bit and 64-bit versions of
init_memory_mapping(), ifdef configuration specific setup code
in the function.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-10-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>