Fix assertion violation in return_single_freelist in child process

Fix assertion violation in return_single_freelist in child process
(Cherry-pick commit cc6d375 from 'release-7_6' branch.)

Issue #171 (bdwgc).

GC_destroy_thread_local cannot be called from a thread other than
the current one.

* pthread_support.c [CAN_HANDLE_FORK && THREAD_LOCAL_ALLOC]
(GC_remove_all_threads_but_me): Do not call GC_destroy_thread_local;
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* win32_threads.c [CAN_HANDLE_FORK && THREAD_LOCAL_ALLOC]
(GC_remove_all_threads_but_me): Likewise.
* thread_local_alloc.c [THREAD_LOCAL_ALLOC] (GC_destroy_thread_local):
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