Ln func discrepancies across arch

## Description

When using `Context#Ln()` func we can observe different results depending on the system's arch, for example with the sample code below:

```go
var decCtx = &apd.Context{
Precision: 34,
MaxExponent: apd.MaxExponent,
MinExponent: apd.MinExponent,
Traps: apd.DefaultTraps,
Rounding: apd.RoundDown,
}
i, _, _ := apd.NewFromString("1.6285091944505809264504560045920167")
var r apd.Decimal
decCtx.Ln(&r, i)
fmt.Println(r.String())
```

The output on arm64 is `0.4876649916811116824516548471782886` and `0.4876649916811116824516548471782887` on amd64.

By investigating it seems the root cause is linked to the usage of `math.Log()` [here](https://github.com/cockroachdb/apd/blob/9cfbeb8698620942481da9e7fb4d46b5ac4df03d/context.go#L755) which is dependent on the system arch.

Is this difference expected? Is there a mean to mitigate this and have consistent results across arch?