Emulate the "ADD reg, r/m" instruction (opcode 03H).

OVMF's flash variable storage is using add instructions when indexing
the variable store bootrom location.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Reviewed by: rgrimes
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D19975