Use yield, not wfe, for asm_volatile_pause on ARM and ARM64

Use yield, not wfe, for asm_volatile_pause on ARM and ARM64

Summary:Found this hunting down LockFreeRingBuffer iOS perf problems. `wfe` and `yield` are similar, but `yield` is the instruction meant for "hey, I'm doing a spinlock", whereas `wfe` tries to wait for certain processor events. See http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204j/Cjafcggi.html

Conveniently, it's the same instruction on 32-bit and 64-bit ARM.

Reviewed By: yfeldblum

Differential Revision: D2986160

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