vmm: Make vmmops declarations more consistent

• On amd64, make vmmops_* functions globally visible, as some will be called from machine-independent code in the future.
• On arm64 and riscv, move declarations to vmm.h, since they're supposed to be generic across different VMM backends (only amd64 has more than one backend).
• Make the declaration macros consistent with each other.
• On amd64, make the function typedef names consistent with the corresponding ifunc names.

No functional change intended.

Reviewed by: corvink
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53011

(cherry picked from commit e9182981f7feeb97ee440bb9c1db9a1bc5309b92)