arm64: Add a VM_FREELIST_DMA32 freelist

When booting a KMSAN kernel on an Ampere Altra, I've seen some boot time
hangs when the XHCI controller driver attempts to allocate memory for
32-bit DMA. The system boots fine with a GENERIC kernel; I believe that
the additional memory requirements of KMSAN push it over the edge. The
system has a bit less than 2GB of RAM below the 4GB boundary.

Allocate a new freelist to segregate memory below 4GB, as we do on
amd64, so that such memory allocation failures are less likely to occur.

Reviewed by: alc
MFC after: 1 month
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D43503