Make extend_size_map() static

Make extend_size_map() static
(code refactoring)

* include/private/gc_priv.h (GC_extend_size_map): Remove prototype.
* malloc.c (GC_extend_size_map): Move the function definition from
misc.c; change GC_INNER to STATIC; reformat code; remove j and
much_smaller_than_i local variables; add assertions that I_HOLD_LOCK
and GC_size_map[i]==0; remove comment that GC_size_map[i] is expected
to be zero.
* malloc.c (GC_generic_malloc_inner): Capitalize the first letter of
the description comment.
* misc.c (GC_extend_size_map): Remove the definition.