We can't grab the VM Lock in free functions

This is due to the way MMTK frees objects, which is on another native thread.
Due to this, there's no `ec` so we can't grab the VM Lock.

This was causing issues in release builds of MMTK on CI like:

```
  /home/runner/work/ruby/ruby/build/ruby(sigsegv+0x46) [0x557905117ef6] ../src/signal.c:948
  /lib/x86_64-linux-gnu/libc.so.6(0x7f555f845330) [0x7f555f845330]
  /home/runner/work/ruby/ruby/build/ruby(rb_ec_thread_ptr+0x0) [0x5579051d59d5] ../src/vm_core.h:2087
  /home/runner/work/ruby/ruby/build/ruby(rb_ec_ractor_ptr) ../src/vm_core.h:2036
  /home/runner/work/ruby/ruby/build/ruby(rb_current_execution_context) ../src/vm_core.h:2105
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor_raw) ../src/vm_core.h:2104
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor) ../src/vm_core.h:2112
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor) ../src/vm_core.h:2110
  /home/runner/work/ruby/ruby/build/ruby(vm_locked) ../src/vm_sync.c:15
  /home/runner/work/ruby/ruby/build/ruby(rb_vm_lock_enter_body) ../src/vm_sync.c:141
  /home/runner/work/ruby/ruby/build/ruby(rb_vm_lock_enter+0xa) [0x557905390a5a] ../src/vm_sync.h:76
  /home/runner/work/ruby/ruby/build/ruby(fiber_pool_stack_release) ../src/cont.c:777
  /home/runner/work/ruby/ruby/build/ruby(fiber_stack_release+0xe) [0x557905392075] ../src/cont.c:919
  /home/runner/work/ruby/ruby/build/ruby(cont_free) ../src/cont.c:1087
  /home/runner/work/ruby/ruby/build/ruby(fiber_free) ../src/cont.c:1180
```

This would have ran into an assertion error in a debug build but we don't run debug builds of MMTK on Github's CI.

Co-authored-by: john.hawthorn@shopify.com

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