Move conditional GC_need_to_lock setting to gc_locks.h

Move conditional GC_need_to_lock setting to gc_locks.h
(code refactoring)

* include/private/gc_locks.h [GC_PTHREADS && !THREAD_LOCAL_ALLOC
&& !USE_PTHREAD_LOCKS && !GC_ASSERTIONS] (UNCOND_LOCK): Remove
GC_ASSERT(I_DONT_HOLD_LOCK()) because it is no-op (when assertion
checking is off).
* include/private/gc_locks.h [THREADS] (set_need_to_lock): New macro.
* pthread_support.c [!GC_ALWAYS_MULTITHREADED]
(GC_allow_register_threads, WRAP_FUNC(pthread_create)): Replace
GC_need_to_lock=TRUE to set_need_to_lock().
* win32_threads.c [!GC_ALWAYS_MULTITHREADED]
(GC_allow_register_threads, GC_CreateThread, GC_init_parallel):
Likewise.
* win32_threads.c [!MSWINCE && !CYGWIN32 && !GC_ALWAYS_MULTITHREADED]
(GC_beginthreadex): Likewise.
* win32_threads.c [GC_PTHREADS && !GC_ALWAYS_MULTITHREADED]
(GC_pthread_create): Likewise.