[System.Numerics] Add support to build with Mono

Required for https://github.com/mono/mono/pull/1433
Yes, Mono today also only supports the format specifiers used in decimalFmt: https://github.com/mono/mono/blob/f8a517624de3fdfc070d2f0660fce1b62d41bc4c/mcs/class/System.Numerics/System.Numerics/BigInteger.cs#L1458-L1471