vmm: Make vmmops declarations more consistent

[markj]

Summary

• 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.