Put the emulator_x86_arm board back

ATV still needs it. ATV will make a local copy
of this board (as the only user) and after that
this change should be reverted.

Bug: 305610937
Test: presubmit
Change-Id: I873a49d0c61b2e9711a33d0ac407d4341601af61
Signed-off-by: Roman Kiryanov <rkir@google.com>