mm: Kconfig: add HAVE_LOW_MEMORY_KILLER

When a low memory killer is available either as a
user/kernel space module, we do not prefer the OOM
killer to trigger early. Add a config to avoid
premature OOMs in the presence of such low memory
killers.

Change-Id: I27f51e347aa7ba559e54753e9962e5ebc67c9ce7
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>