Fix an age-old bug with FromFloat handling of negatives.

[robshakir]

 * (M) pkg/yang/types_builtin(_test)?.go
    - This change squashes a bug that has existed for a long time
      in `FromFloat`, but only shows up on arm64. I finally have an
      arm64 machine and so found the issue. It was originally reported
      in https://github.com/openconfig/ygot/issues/766. The issue is
      that uint64 of a negative float64 is undefined  -- and on arm64
      on darwin returns 0, which made some test cases fail only on this
      architecture.

[coveralls]

coverage: 77.848% (+0.005%) from 77.843%
when pulling 69235d4 on fix-negatives
into dfbf7ba on master.

[wenovus]

Nice find, thanks.

Interesting to know that Go also has undefined-ish behaviour for non-sensical code: https://go.dev/issue/52943