encoding/binary: replace constant literals with named constant (cleanup)
encoding/binary: replace constant literals with named constant (cleanup)

Follow-up on https://golang.org/cl/247120.

Brought to my attention by Luke McCoy.

Change-Id: I4530c96fb164d23b0ce5311f2cecb1964f2dea74
Reviewed-on: https://go-review.googlesource.com/c/go/+/249837
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

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