[vm] Fix cross-word arm.

There seems to be a packing difference between arm and simarm_x64.

TEST=ci
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-release-simarm_x64-try,vm-aot-linux-debug-simarm_x64-try,vm-aot-android-debug-arm_x64-try,vm-aot-android-release-arm_x64-try
Change-Id: I2141adfb67301995f994a3de419e4316f855e4c4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442505
Reviewed-by: Siva Annamalai <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google