[thread] Only register a dead letter if the current NSThread is non-null.

This breaks all embedder that don't link with CoreFoundation. In mono's case, monodis.
[boehm] Fix memory management for toggleref.

Replace GC_free with GC_INTERNAL_FREE. The former might take the GC lock
and deadlock us.

Replace GC_INTERNAL_MALLOC with GC_INTERNAL_MALLOC_IGNORE_OFF_PAGE as
we don't need the rest of the GC messing around with the toggleref array.