Okay, I went a little further from the last time. I have been able to compile a basic rtl, by making some little fixes to the linux' one. I think it could be the right way, because the gnu-arm crosscompiler, usually used for gba, works without changes from the basic linux rtl.
With this rtl the compiler works without errors. The next step will be to make a startup code (prt0.as), in order to put all things to the right place. The biggest problem in this step is to make to cohabit the fpc initialization code and the gba's one. I'm not comfortable with asm, so the work proceeds slowly. If someone is interested, i can make a little guide to show my steps until this point.

PS. I forgot often to say "sorry for my english"... well, if someone can't understand my words, we can try with french or italian