Platforms.txt: Add and update more systems

- update i686/pc/linux-gnu, gcc 2.7.2
- update powerpc/apple/darwin6.8, gcc 3.1
- add powerpc/apple/darwin9.8.0, gcc 4.0.1

Thanks to Götz Hoffart <goetz@hoffart.de>!
Platforms.txt: Update x86_64/unkn./linux-gnu, clang 3.x

- Update x86_64/unknown/linux-gnu, clang 3.2 => clang 3.3
Platforms.txt: Update powerpc/apple/darwin7.9.0

- Update powerpc/apple/darwin7.9.0, gcc 3.3

Thanks to Götz Hoffart <goetz@hoffart.de>!
Platforms.txt: Update i686/unknown/gnu0.5