Fix MS VC redefinition warning for functions declared with GC_ATTR_MALLOC

Fix MS VC redefinition warning for functions declared with GC_ATTR_MALLOC

* dbg_mlc.c (GC_debug_malloc, GC_debug_malloc_ignore_off_page,
GC_debug_malloc_atomic_ignore_off_page, GC_debug_malloc_stubborn,
GC_debug_malloc_atomic, GC_debug_strdup, GC_debug_strndup,
GC_debug_wcsdup, GC_debug_malloc_uncollectable,
GC_debug_malloc_atomic_uncollectable,
GC_debug_generic_or_special_malloc, GC_debug_malloc_replacement): Add
GC_ATTR_MALLOC attribute to API function definition (to avoid MS VC
warning about redefinition of a symbol previously declared with
__declspec).
* fnlz_mlc.c (GC_finalized_malloc): Likewise.
* gcj_mlc.c (GC_gcj_malloc, GC_debug_gcj_malloc,
GC_gcj_malloc_ignore_off_page): Likewise.
* malloc.c (GC_generic_malloc, GC_malloc_atomic, GC_malloc,
GC_malloc_uncollectable): Likewise.
* mallocx.c (GC_generic_or_special_malloc,
GC_generic_malloc_ignore_off_page, GC_malloc_ignore_off_page,
GC_malloc_atomic_ignore_off_page, GC_malloc_many, GC_memalign,
GC_malloc_atomic_uncollectable, GC_strdup, GC_strndup, GC_wcsdup):
Likewise.
* stubborn.c (GC_malloc_stubborn): Likewise.
* thread_local_alloc.c (GC_malloc, GC_malloc_atomic, GC_gcj_malloc):
Likewise.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page,
GC_calloc_explicitly_typed): Likewise.