init: don't touch mmap_rnd_compat_bits on 64-bit only builds

mmap_rnd_compat_bits is for address space randomization of 32-bit
applications on 64-bit system. Configuring it is not only unnecessary
for 64-bit "only" builds, but also can cause a boot failure if the
kernel is built without CONFIG_COMPAT which is the case for Microdroid.

Use ro.product.abilist32 to determine whether 32-bit applications are
supported and if not, don't configure it, but mmap_rnd_bits.

Bug: 237950549
Test: run Microdroid with the kernel built with aosp/2153639

Change-Id: Ifca6fa02f14ad4c7d8f9b2ab8852494c12945c3a