Add additional BigInt constructors for [u]int(64) args (where c_[u]long is expected).

A few tests in modules/standard/gmp were failing on linux32 to resolve calls
like "BigInt(bool, uint(64)" or "BigInt(int(64))". The intended constructors
were "BigInt(init2:bool, nbits:c_ulong)" and "BigInt(num:c_long)" respectively.

Create the [u]int(64) versions and cast from [u]int(64) to c_[u]long.