Platforms.txt: Add and update systems

- Add sparc/unknown/openbsd5.5
- Update powerpc/apple/darwin7.9.0
- Update x86_64/unknown/linux-gnu, Open64
- Update x86_64/unknown/linux-gnu, Sun C 5.12/Solaris Studio 12.3
- Update x86_64/unknown/linux-gnu, tcc 0.9.25

Thanks to Götz Hoffart <goetz@hoffart.de>!