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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

sorry, I wasn't able to figure out how to fix-forward these failures quickly. Suggest revert in https://github.com/llvm/llvm-project/pull/78329 so that we can get the 32b arm build bot back online

https://github.com/llvm/llvm-project/pull/78192