doc/Platforms.txt: Update some systems to ngIRCd 21

- update i686/pc/cygwin, gcc 4.8.2
- update x86_64/apple/darwin13.0.0, Apple clang 5.0
- update x86_64/unknown/freebsd8.4, gcc 4.2.1
- update x86_64/unknown/freebsd9.1, gcc 4.2.1
- update x86_64/unknown/linux-gnu, gcc 4.8.2
- update x86_64/unknown/linux-gnu, Open64
- update x86_64/unknown/openbsd4.8, gcc 4.2.1
- update x86_64/unknown/openbsd5.1, gcc 4.2.1