Revert "[libc] Refactor `BigInt`" (#87612)

Reverts llvm/llvm-project#86137

Some aarch64 compilers seem to consider that `uint128_t` is not
`is_trivially_constructible` which prevents `bit_cast`-ing.

GitOrigin-RevId: 12735916bd3a63aa9f316af8eebfe9420cfec489
12 files changed