arm64: Add pmap integration for KMSAN

[markj]

Summary

• In pmap_bootstrap_san(), allocate the root PTPs for the shadow maps. (For KASAN, this is done earlier since we need to do some special bootstrapping for the kernel stack.)
• Adjust ifdefs to include KMSAN.
• Expand the shadow maps when pmap_growkernel() is called.

Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.