[libc][NFC] Make EXPONENT_BIAS int32_t (#75046)

`EXPONENT_BIAS` is almost always used with signed arithmetic. Making it
an `int32_t` from the start reduces the chances to run into
implementation defined behavior (cast from unsigned to signed is
implementation-defined until C++20).


https://en.cppreference.com/w/cpp/language/implicit_conversion#:~:text=If%20the%20destination%20type%20is%20signed,arithmetic%20overflow%2C%20which%20is%20undefined).