[vm] Triage finalizer flakiness / crashes

[dcharkes]

flakiness dashboard

current results

Various failures:

• [x] vm/cc/Finalizer_CrossGen_xx assert
  • https://github.com/dart-lang/sdk/issues/48715
• [x] vm/dart_2/finalizer/finalizer_isolate_groups_run_gc_test in optcounter mode RTE
  • log
• [x] ffi_2/vmspecific_native_finalizer_isolate_groups_test segfault
  • log
• [x] ffi_2/vmspecific_native_finalizer_isolate_groups_test RTE
  • log
  • https://github.com/dart-lang/sdk/issues/48674
• [x] UBSAN failure with function signature
  • log

TODO: triage and fix.

Update: https://dart-review.googlesource.com/c/sdk/+/240043 should address all of these.

[dcharkes]

@mkustermann and I discussed whether any of the fixes are needed for cherry picking, but the answer is no. See CL description for more info.