core: pause in guest loops that poll the system counter

Title 1's main thread reads CNTPCT ~17 million times a second while it waits for the
GPU (~23% of its samples are the counter read). On an SMT core that spin takes
resources from the sibling thread, often the GPU thread. Two reads within ~1.7 us of
each other now execute two PAUSE instructions first; the guest sees the same values.
EDEN_NX2_CNTPCT_PAUSE=0 turns it off.

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