ARM: fix nommu builds with 48be69a02 (ARM: move signal handlers into a vdso-like page)

Olof reports that noMMU builds error out with:

arch/arm/kernel/signal.c: In function 'setup_return':
arch/arm/kernel/signal.c:413:25: error: 'mm_context_t' has no member named 'sigpage'

This shows one of the evilnesses of IS_ENABLED().  Get rid of it here
and replace it with #ifdef's - and as no noMMU platform can make use
of sigpage, depend on CONIFG_MMU not CONFIG_ARM_MPU.

Change-Id: I5875b23e7c076aeb8240081bd48a4c8516b612cb
Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Git-commit: 8c0cc8a5d90bc7373a7a9e7f7a40eb41f51e03fc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[joonwoop@codeaurora.org: fixed merge conflict]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>