Fix divide-by-zero panic when ASLR is enabled and superpages disabled

When locating the anonymous memory region for a vm_map with ASLR
enabled, we try to keep the slid base address aligned on a superpage
boundary to minimize pagetable fragmentation and maximize the potential
usage of superpage mappings. We can't (portably) do this if superpages
have been disabled by loader tunable and pagesizes[1] is 0, and it
would be less beneficial in that case anyway.

PR: 253511

(cherry picked from commit 41032835dc2d489ec7841d7529f74f6389329cd3)