enable sbrk() for everyone
The sbrk() function can be used to merely query sizes and for that, no-mmu
works the same as mmu. It can also sometimes increase data segments on
no-mmu systems provided the trailing memory is free. So, there is no real
reason to exclude this function for no-mmu ports.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>