[v3] default linux allocator

[Loren James Rittle]

> Yo dudes... this switches this default allocator for linux to
> mt_allocator. I'd like to try this out on mainline, and then possibly
> move this to the gcc-3_4-branch after a review of gcc-testresults.

> Sound like a good idea? Barring objections, I'll do this tomorrow.

Unless you reworked the magic since I last studied it in detail, I
think that changing the default allocator changes the ABI.  I.e. code
compiled with 3.4.0 and 3.4.1 will not forward link against code
compiled with 3.4.2.

If you added all the indirection required to address this issue,
please ignore my observation.  Likewise, I can't remember if we care.

Regards,
Loren