Rename ATOMIC_UNCOLLECTABLE to GC_ATOMIC_UNCOLLECTABLE

Rename ATOMIC_UNCOLLECTABLE to GC_ATOMIC_UNCOLLECTABLE
(code refactoring)

* Makefile.direct (CFLAGS): Add GC_ prefix to ATOMIC_UNCOLLECTABLE.
* SMakefile.amiga: Likewise.
* configure.ac (enable_atomic_uncollectible): Likewise.
* dbg_mlc.c (GC_print_obj, GC_debug_malloc_atomic_uncollectable,
GC_debug_free, GC_debug_realloc): Likewise.
* doc/README.Mac: Likewise.
* doc/README.macros (ATOMIC_UNCOLLECTABLE): Likewise.
* extra/AmigaOS.c (GC_amiga_allocwrapper_any): Likewise.
* include/new_gc_alloc.h: Likewise.
* include/private/gc_priv.h (struct _GC_arrays, AUNCOLLECTABLE):
Likewise.
* mallocx.c (GC_auobjfreelist_ptr, GC_generic_or_special_malloc,
GC_malloc_atomic_uncollectable): Likewise.
* mark.c (GC_obj_kinds, GC_N_KINDS_INITIAL_VALUE): Likewise.
* include/private/gc_priv.h (GC_ATOMIC_UNCOLLECTABLE): Define if
ATOMIC_UNCOLLECTABLE defined (for compatibility with the clients that
do not use GC makefiles).