Add support for low resolution SMP kernel profiling.

Add support for low resolution SMP kernel profiling.

  - A nonprofiling version of s_lock (called s_lock_np) is used
    by mcount.

  - When profiling is active, more registers are clobbered in
    seemingly simple assembly routines. This means that some
    callers needed to save/restore extra registers.

  - The stack pointer must have space for a 'fake' return address
    in idle, to avoid stack underflow.