Permit implementations to return canonical NaNs from arithmetic. (#660)

* Permit implementations to return canonical NaNs from arithmetic.

RISC-V and ARM in "default NaN" mode do not propagate NaN values in
arithmetic operations. Instead, they return a "canonical" NaN.

IEEE 754 doesn't require NaN propagation; it's merely a "should".

In order to ensure that wasm doesn't accidentally over-burden support
for the above and other potential future IEEE-754-conforming platforms,
this patch changes the NaN rules to permit either behavior.

* Add the IEEE 754 section number for the NaN propagation recommendation.

* Avoid "typically".

* Reword the NaN conversion rules for clarity.

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