Add support for targetting i686 GNU/Linux binaries

[a1batross]

To match linux64 it's called linux32.

[a1batross]

I can agree there is no sense to support 32-bit x86 systems on Linux, where barely any distributions support it, but I made it for my own game development (and binary compatibility) purposes and hope it might be useful for somebody else too.

[BtbN]

This doesn't appear to work.
It's failing to build openssl for me, and probably a bunch of later scripts will fail too.

[a1batross]

Ah, right, I'm building myself a very minimalistic ffmpeg that only do software decoding and don't pull all these dependencies. I will try to look into that later.