Replace pointer relational comparisons with non-pointer ones

Replace pointer relational comparisons with non-pointer ones

* src/atomic_ops_malloc.c (initial_heap_lim): Remove static variable
(which holds a constant).
* src/atomic_ops_malloc.c (get_chunk): Replace "goto" with for/break;
remove "my_lim" local variable; cast AO_compare_and_swap_acquire()
unused result to void; replace less-or-equal comparison of pointers
with comparison of ptrdiff_t value.
* src/atomic_ops_malloc.c (add_chunk_as): Update comment; remove "sz"
argument; replace "first", "limit", "p", "next" pointer local variable
with "ofs", "limit" and "sz" variables of size_t; add assertion on
"sz" value; replace less-or-equal comparison of pointers with
comparison of size_t values.
* src/atomic_ops_malloc.c (AO_malloc): Remove "adj_sz" local variable;
do not pass adj_sz argument to add_chunk_as.