Minor code refactoring of GC fork handlers (remove GC prefix)

Minor code refactoring of GC fork handlers (remove GC prefix)

* pthread_support.c (GC_fork_prepare_proc, GC_fork_parent_proc,
GC_fork_child_proc): Declare as "static" instead of STATIC; remove
"GC_" prefix.
* win32_threads.c (GC_fork_prepare_proc, GC_fork_parent_proc,
GC_fork_child_proc): Likewise.
* pthread_support.c (GC_fork_parent_proc): Refine comment.
* pthread_support.c (GC_thr_init): Remove "GC_" prefix for
fork_prepare/parent/child_proc (only if CAN_HANDLE_FORK).
* win32_threads.c (GC_thr_init): Likewise.