sim posix: make simulator shut down cleanly on ctrl-c

This allows the simulator executable to write its gcov
profiling data to disk at exit before dying.
sim posix: enable SIGFPE handler to spot math bugs instantly

This signal will kill the sim as soon as a floating point
exception occurs.  Run the sim with gdb to point out the
error instantly like this:
  gdb -x flight/Project/gdb/simposix \
         build/sim_posix_revolution/sim_posix_revolution.elf

gdb will halt on SIGFPE and you will have access to the
error context.