stand: Add back missing EFIAPI define

EFIAPI has to be defined correctly for amd64, or things won't boot
because it uses a different API than we normally use. Normally, this
only affects amd64, since all the other archs are basically nothing.
Tested on: amd64, aarch64 and armv7 (the frist two by markj and I with
differnet test setups).

Fixes: 43b8edb32051
Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53799