sim: signal: mark signal callback funcs as noreturn since they don't return

sim: signal: mark signal callback funcs as noreturn since they don't return

All funcs already call other funcs that don't return.  The mips port is
the only exception because its generic exception handler can return in
the case of normal exceptions.  So while the exceptions its signal handler
triggers doesn't return, we can't express that conditional logic.  So add
some useless abort calls to make the compiler happy.