sim: mcore: drop sbrk support

sim: mcore: drop sbrk support

The sbrk syscall assumes the sbrk region starts after the bss and the
current implementation requires a bss section to exist.  Since there
is no requirement for programs to have a bss in general, we want to
drop this check.  However, there is still the sbrk syscall that wants
to know about the region.

Since libgloss doesn't actually use the sbrk syscall (it implements
sbrk in its own way), and the sim really shouldn't enforce a specific
memory layout on programs, lets simply delete sbrk support.  Now it
always returns an error.