security: select correct default LSM_MMAP_MIN_ADDR on ARM.

The default for this is universally set to 64k, but the help says:

   For most ia64, ppc64 and x86 users with lots of address space
   a value of 65536 is reasonable and should cause no problems.
   On arm and other archs it should not be higher than 32768.

The text is right, in that we are seeing selinux-enabled ARM targets
that fail to launch /sbin/init because selinux blocks a memory map.
So select the right value if we know we are building ARM.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: James Morris <jmorris@namei.org>