You'll have to adapt morecore to your needs: for instance, by replacing sbrk.
I guess, this problem was already discussed in this mailling list at least couple of times :}
I'd like to expand on that a little bit. In general, You'll have to deal with linker scripts and mapping properly the RAM. Also, there are plenty of MCUs with terribly segmented RAM, in chunks of 16-32 whatever kbytes. Which... makes sbrk() implementation a bit tricky.
All in all, that's the generic way dealing with more or less custom or just various embedded devices like ARM boards.