refactor gcc4, add version hints

gcc4 is now a meta script that points to gcc454 as default.
this is the compiler that was tested against all supported archs.
add experimental gcc 4.7.2
add experimental gcc 4.6.3
add experimental gcc 4.4.7
adapt gcc 4.2.4 to create a version specific link
adapt gcc 3.4.6 to create a version specific link