Free microcode memory later.

With lockless DI, pmap_remove() requires operational thread lock,
which is initialized at SI_SUB_RUN_QUEUE for thread0. Move it even
later where APs are started, the moment after which other boot memory
like trampoline stacks is already being freed.

Reported by: gtetlow
Sponsored by: The FreeBSD Foundation
MFC after: 30 days