malloc: Trim unused arenas on thread exit

[Florian Weimer]

I tried the attached patch to trim unused arenas on thread exit.  The 
trimming actually happens (the heap consolidation is visible in the 
malloc_info output from tst-malloc_info), but the arena heaps aren't 
deallocated.

I think trimming unused arenas as much as possible is a good heuristics 
to minimize RSS, so getting this to work might be worthwhile.

Thanks,
Florian
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