This patch fixes a runtime issue I encountered with the AMD GCN Unified 
Shared Memory implementation.

We were using regular malloc'd memory configured into USM mode, but 
there were random intermittent crashes. I can't be completely sure, but 
my best guess is that the HSA driver is using malloc internally from the 
same heap, and therefore using memory on the same page as the offload 
kernel. What I do know is that I could make the crashes go away by 
simply padding the USM allocations before and after.

With this patch USM allocations are now completely separated from the 
system heap. The custom allocator is probably less optimal is some 
use-cases, but does have the advantage that all the metadata is stored 
in a side-table that won't ever cause any pages to migrate back to 
main-memory unnecessarily. It's still possible for the user program to 
use USM memory in a way that causes it to thrash, and this might have 
been the ultimate cause of the crashes, but there's not much we can do 
about that here.

I've broken the allocator out into a new file because I anticipate it 
being needed in more than one place, but I didn't put full 
data-isolation on it yet.

I'll rebase, merge, and repost all of the OpenMP memory patches sometime 
soonish.

Andrew