arm64: Implement cpu_relax as yield
ARM64 has the yield nop hint which has the intended semantics of cpu_relax. Implement. The immediate application is ARM CPU emulators. An emulator can take advantage of the yield hint to de-prioritise an emulated CPU in favor of other emulation tasks. QEMU A64 SMP emulation has yield awareness, and sees a significant boot time performance increase with this change. Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com> Acked-by: NWill Deacon <will.deacon@arm.com> Signed-off-by: NWill Deacon <will.deacon@arm.com>
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