doc/Platforms.txt: Add and update more systems

- add i386/unknown/netbsdelf1.5.2, egcs-1.1.2
- update i386/unknown/openbsd5.3, gcc 4.2.1
- add i386/unknown/openbsd5.4, gcc 4.2.1
- add x86_64/unknown/linux-gnu, nwcc 0.8.2
- update x86_64/unknown/linux-gnu, tcc 0.9.25

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