[libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (#78192)

This patch removes the `FPBits` specialization for x86 Extended Precision by moving it up to `FPRep`.
It also introduces enums (`Exponent`, `BiasedExponent` and `Significand`) to represent the exponent and significant parts of the floating point numbers. These enums are used to construct and observe floating point representations.

Additionally, we remove `LongDoubleBits.h` that is now unnecessary.