Portaudio backend doesn't work on x64 because of hardcoded library name

[radgeRayden]

SoLoud version, operating system, backend used, any other potentially useful information:

I'm using master under an Ubuntu derivative (x64). I wanted to try the portaudio backend, but it wouldn't initialize due to a dll error. Then I looked into the source a little bit and found out that the library name is hardcoded for x86. After changing this on my end to "libportaudio.so", things just worked.

This would be a PR if I knew how to properly make a check for a x64 build, and if you give me orientation I'd be happy to do it still.