Add NIOS2 double conversion detection, fixes compile errors

This change gets Foundation compiling for Nios2. Previously, double conversion on this platform was not getting detected at all.

I tested the double conversion process on a Nios2 Linux system with this program:

```c++
#include <iostream>
int main() {
    double num = 89255.0;
    double denom = 1e22;

    if(num / denom == 89255e-22) {
        std::cout << "Double operations appear correct: " << num / denom << " == " << 89255e-22 << std::endl;
    } else {
        std::cout << "Double operations appear wrong: " << num / denom << " != " << 89255e-22 << std::endl;
    }

}
```

It outputs that the operations appear correct.

After this change, I was able to build for the Nios2 with:

    $ ./configure --config=NIOS2-Linux --no-fpenvironment --omit=Data,Data/ODBC,Data/MySQL,Crypto,NetSSL_OpenSSL,MongoDB
    $ make