Do some minor critical path performance improvements in the scheduler

Do some minor critical path performance improvements in the scheduler
and at the user/system boundary.  Avoid some unnecessary segment prefix ops,
remove some unnecessary memory ops by using more optimal critical
section inlines, and use 32 bit arithmatic instead of 64 bit arithmatic
when calculating system tick overheads in userret().

This saves a whopping 5ns worth of syscall overhead, which just proves
how silly I am sometimes.