spin loop primitives for busy waiting

Current busy-wait loops are implemented by repeatedly calling cpu_relax()
to give an arch option for a low-latency option to improve power and/or
SMT resource contention.

This poses some difficulties for powerpc, which has SMT priority setting
instructions (priorities determine how ifetch cycles are apportioned).
powerpc's cpu_relax() is implemented by setting a low priority then
setting normal priority. This has several problems:

 - Changing thread priority can have some execution cost and potential
   impact to other threads in the core. It's inefficient to execute them
   every time around a busy-wait loop.

 - Depending on implementation details, a `low ; medium` sequence may
   not have much if any affect. Some software with similar pattern
   actually inserts a lot of nops between, in order to cause a few fetch
   cycles with the low priority.

 - The busy-wait loop runs with regular priority. This might only be a few
   fetch cycles, but if there are several threads running such loops, they
   could cause a noticable impact on a non-idle thread.

Implement spin_begin, spin_end primitives that can be used around busy
wait loops, which default to no-ops. And spin_cpu_relax which defaults to
cpu_relax.

This will allow architectures to hook the entry and exit of busy-wait
loops, and will allow powerpc to set low SMT priority at entry, and
normal priority at exit.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>