std: Improve spinloop hint

* Add a yield pattern for PowerPC64
* Fix compile error on pre-v6 ARM targets
* Use isb instead of yield on AArch64 to give the CPU a chance to enter
  low-power states.
* Make the hint an inline function, the call overhead can be avoided.
807bb7c03fLemonBoy committed on 4/30/2021, 3:11:03 PMparent55c58f2
1 file changedLine totals unavailable