Set proper CPU variant for aosp_arm

System image of aosp_arm products is the new GSI in Pi.
Its arch variants need to be the same as the legacy GSI built
with aosp_arm_ab so it can pass related CTS/VTS tests.

Bug: 80401108
Test: $ lunch aosp_arm-userdebug; m -j; emulator # booted OK
      $ lunch aosp_arm-userdebug; m -j cts

Change-Id: I29fffca3e02a2251913a327b54640fc622e77a8d