Haiku switches system allocator to rpmalloc

As of hrev53136, we've finally replaced the aging hoard2 with a shiny new mmap-based allocator - mjansson's rpmalloc. Thanks to @pulkomandy and @mmlr for helping out with that work! The main benefit here will be on 64-bit Haiku, as applications will now (finally) be able to use more than 1.5GB of RAM each, a limitation of the old allocator. But there are some pretty nice (10-15%) performance benefits over the old allocator, too. More of the technical details can be found in the commit message 5, but essentially the only thing to be concerned about is if things start suddenly crashing more often. It's already known to exacerbate a few pre-existing WebKit crashes (mostly around Google Maps or the like, which were already so unstable as to be unusable anyway).