[libc][NFC] Simplify FPBits

[gchatelet]

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.

[gchatelet]

I'm pushing all of the commits so it's easier to review.
The goal here is to get closer to https://godbolt.org/z/9fxf1dea5.