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.