You can adjust that, as Emerson said, just look for guide on setting ICC as your Gentoo compiler, the procedure should be mostly the same for any other compiler. But be warned that it won't be *that* simple. Some (or even a lot of) packages will fail to compile, and the kernel won't probably compile against anything that's not gcc unless you patch it.
If there are some other problems in the produced object code and/or the ABI and linking you will be, basically, on your own. The place to seek for support on this would be the tcc forums, mailing lists, or whatever they use. Gentoo doesn't provide support for alternative toolchains (the thing is already complex enough as it is...).