Add support of GC_ATTR_MALLOC for MS VS

Add support of GC_ATTR_MALLOC for MS VS

* include/gc.h (GC_malloc, GC_malloc_atomic, GC_strdup, GC_strndup,
GC_malloc_uncollectable, GC_malloc_stubborn, GC_memalign,
GC_malloc_ignore_off_page, GC_malloc_atomic_ignore_off_page,
GC_malloc_atomic_uncollectable, GC_debug_malloc_atomic_uncollectable,
GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_strdup,
GC_debug_strndup, GC_debug_malloc_uncollectable,
GC_debug_malloc_stubborn, GC_debug_malloc_ignore_off_page,
GC_debug_malloc_atomic_ignore_off_page, GC_debug_malloc_replacement,
GC_debug_realloc_replacement, GC_wcsdup, GC_debug_wcsdup): Move
GC_ATTR_MALLOC and GC_ATTR_ALLOC_SIZE from the function declaration
tail to the beginning (just after GC_API) for compatibility with MS VC.
* include/gc_gcj.h (GC_gcj_malloc, GC_debug_gcj_malloc,
GC_gcj_malloc_ignore_off_page): Likewise.
* include/gc_config_macros.h (GC_ATTR_MALLOC): Implement for
MS VC (Visual Studio) v14+.