[reland][libc] Make BigInt bit_cast-able to compatible types (#74862)

Fix #74258
This is a reland of #74837, the error went unnoticed because it compiles
fine on
clang-16 but not on clang-12 which is the version used on the buildbots.

The fix was to explicitly initialize `BigInt` variables in `constexpr`
operations: `BigInt<Bits, Signed> result(0);` instead of `BigInt<Bits,
Signed> result;`