extend dl-minimal malloc implementation

[Florian Weimer]

On 08/11/2017 02:09 PM, Carlos O'Donell wrote:
> I think we should push this upstream. It allows the full malloc API to be
> used effectively instead of alloca.
> 
> Are we worried about something?

I'm not sure we have consensus to move from alloca to malloc.  The IFUNC
scheduler needs some allocator, but I'm no longer certain the minimal
malloc will work for it because we are switching the malloc
implementation as part of the relocation.

There is some code to ensure that the dynamic linker itself is relocated
last, but I don't know how effective it is in practice.

I'm also concerned that without block merging on free, we can't return
any allocated memory to the operating system once the initial relocation
is finished.

Florian