[libc][NFC] Simplify `FPBits` (#76835)

This patch reduces the scope of `FPBits` exported variables and
functions.
It also moves storage up into `FPRep` and tries to make the default and
specialized versions of `FPBits` more uniform.

The next step is to move the specialization from `FPBits` to `FPRep` so
we can manipulate floating point representations through `FPType` 
alone - that is - independently from the host architecture.