maxinsns flag does not work when ncpus > 1

[kabylkas]

Dromajo does not respond to maxinsns variable when ncpus > 1

To reproduce:

./dromajo --trace 0 --maxinsns 100000 --ncpus 2 [riscv-bin]

[kabylkas]

It seems the issue is related to the unsignedness of maxinsns or the fact that it is common to all cores. Once maxinsns riches zero, the next core in the loop overflows it here: https://github.com/chipsalliance/dromajo/blob/6f68f74a2a3ca26c5460f7864d5769face2c69bd/src/dromajo.cpp#L125

and this condition fails: https://github.com/chipsalliance/dromajo/blob/6f68f74a2a3ca26c5460f7864d5769face2c69bd/src/dromajo.cpp#L127

[kabylkas]

Fixed in #76