i#6938 migrate: Fix observed_migrations assert in schedule_stats

[derekbruening]

Fixes an assert on the new observed_migrations stat added to schedule_stats in PR #7057. These observed_migrations are counted on the destination core, while the scheduler reports migrations away from a source core: so they can differ, causing the assert to fire. Fixed by moving it to only check the aggregated stats across all cores.

Tested on the internal trace where the assert fired before.

Issue: #6938

[derekbruening]

riscv failure is the invalid type before a bundle assert #3320