New API function to get size of object debug header

New API function to get size of object debug header

GC_get_debug_header_size() is exported in addition to
GC_debug_header_size variable.  The usage of the latter is deprecated.

* dbg_mlc.c [CPPCHECK] (GC_start_debugging_inner): Call
GC_noop1(GC_debug_header_size) to suppress a cppcheck warning about
unused symbol.
* dbg_mlc.c (GC_debug_header_size): Add const.
* dbg_mlc.c (GC_get_debug_header_size): New API function definition.
* include/gc_config_macros.h (GC_ATTR_CONST): New macro.
* include/gc_mark.h (GC_debug_header_size): Add GC_ATTR_DEPRECATED and
const; update comment.
* include/gc_mark.h (GC_get_debug_header_size): New API public const
function declaration; move the comment from GC_debug_header_size.
* include/gc_mark.h (GC_USR_PTR_FROM_BASE): Use
GC_get_debug_header_size() instead of GC_debug_header_size.