[atomics.types.float] Align parameters in function declarations

[eisenwave]

Fixes https://github.com/cplusplus/nbballot/issues/891.

The parameter indentation in the subclause is broken in four different ways:

• In the first block with exchange, fetch_max, etc. it looks like constexpr was added without adding then necessary indentation to the second line of the declaration, resulting in misalignment.
• For store_fmaximum and some others, an attempt was made to align the parameters, but unsuccessfully.
• In the definition of fetch_key, there is one space too much indentation, and we forgot to use placeholdernc rather than placeholder.
• In the definition of store_key, we just used two-space indentation rather than trying to align parameters at all.

[tkoeppe]

Perfect, thanks!