It looks like the compilation process just crashes.
Could it be you don't have enough RAM? Building the core setting
unfortunately takes quite a bit of memory, since it's such a gigantic
piece of code.
Try re-running with a swap file maybe? Alternatively, cd into the rakudo
folder that rakudobrew created for you (somewhere under
~/.rakudobrew/moar maybe?) and run the command manually:
perl6.moarvm --setting=NULL --ll-exception --optimize=3 --target=mbc \
--stagestats --output=CORE.setting.moarvm gen/moar/m-CORE.setting
maybe even under GDB.
That should give you a little more detail on what's going on.