Use the 64b inner:monotonize() implementation not the 128b one for aarch64

aarch64 prior to v8.4 (FEAT_LSE2) doesn't have an instruction that guarantees
untorn 128b reads except for completing a 128b load/store exclusive pair
(ldxp/stxp) or compare-and-swap (casp) successfully. The requirement to
complete a 128b read+write atomic is actually more expensive and more unfair
than the previous implementation of monotonize() which used a Mutex on aarch64,
especially at large core counts.  For aarch64 switch to the 64b atomic
implementation which is about 13x faster for a benchmark that involves many
calls to Instant::now().

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