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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Backed out in https://github.com/llvm/llvm-project/commit/337b7715929909fd2bb55069ecb939cf23dbfe52.  I'll stand up the 32b arm build bot to help catch these kinds of things, ASAP. Let me know if you need help testing on the 32b build bot (or the baremetal 32b arm config should work on an x86 host for reproducing the errors). I suspect we may need to improve the BigInt related cast operators added in 1ee6a1e38aa0c3773d892fcd01bb2af8e446e67f.

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