Add alloc_size attribute to GC_generic_malloc

* include/gc_mark.h (GC_generic_malloc,
GC_generic_malloc_ignore_off_page): Add GC_ATTR_ALLOC_SIZE attribute.
Define public GC_GENERIC_OR_SPECIAL_MALLOC and GC_get_kind_and_size

* dbg_mlc.c (GC_debug_generic_malloc,
GC_debug_generic_or_special_malloc): New function.
* mallocx.c (GC_get_kind_and_size): Likewise.
* tests/test.c (test_generic_malloc_or_special): Likewise.
* include/gc_mark.h (GC_generic_or_special_malloc,
GC_debug_generic_or_special_malloc, GC_get_kind_and_size): New API
prototype.
* include/gc_mark.h (GC_GENERIC_OR_SPECIAL_MALLOC): New public macro.
* mallocx.c (GC_generic_or_special_malloc): Turn into API function.
* tests/test.c (reverse_test_inner, run_one_test): Invoke
test_generic_malloc_or_special (to test GC_GENERIC_OR_SPECIAL_MALLOC
and GC_get_kind_and_size).