[libc] Remove UB specializations of type traits for `BigInt` (#84035)

The standard specifies that it it UB to specialize the following traits:
 - `std::is_integral`
 - `std::is_unsigned`
 - `std::make_unsigned`
 - `std::make_signed`

This patch:
 - Removes specializations for `BigInt`
 - Transforms SFINAE for `bit.h` functions from template parameter to
   return type (This makes specialization easier).
 - Adds `BigInt` specialization for `bit.h` functions.
 - Fixes code depending on previous specializations.