vmm: Dynamically allocate a couple of per-CPU state save areas

This avoids bloating the BSS when MAXCPU is large.

No functional change intended.

PR: 269572
Reviewed by: corvink, rew
Tested by: rew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39805

(cherry picked from commit 74ac712f72cfd6d7b3db3c9d3b72ccf2824aa183)