This adds a new atomic type, `atomicbitops.Float64`, which has similar
operations as `atomicbitops.Uint64`. It actually uses `atomicbitops.Uint64`
for storing its bits.
`atomicbitops.Float64` supports `Swap`, `CompareAndSwap`, and `Add`
operations.
This is useful in gVisor's metric library for keeping track of the
sum-of-squared-deviation statistic of distribution metrics.
PiperOrigin-RevId: 537127647
Etienne Perot