return chunks split off by memalign using __bin_chunk instead of free

return chunks split off by memalign using __bin_chunk instead of free

this change serves multiple purposes:

1. it ensures that static linking of memalign-family functions will
pull in the system malloc implementation, thereby causing link errors
if an attempt is made to link the system memalign functions with a
replacement malloc (incomplete allocator replacement).

2. it eliminates calls to free that are unpaired with allocations,
which are confusing when setting breakpoints or tracing execution.

as a bonus, making __bin_chunk external may discourage aggressive and
unnecessary inlining of it.