32bit user mode, want to compile 64bit programs

Hi,

I'm new to gentoo and I've installed 64bit kernel and 32bit userland on 
a PowerMac G5.

Now I want to compile a program as 64bit.

$ gcc -m64 -o hello hello.c
In file included from /usr/include/features.h:400:0,
                  from /usr/include/stdio.h:27,
                  from hello.c:1:
/usr/include/gnu/stubs.h:11:27: fatal error: gnu/stubs-64.h: No such 
file or directory
compilation terminated.
$


I assume, I'm missing the 64bit version of glibc (and maybe other libs).

How do I install 64bit versions of selected libraries? I don't want to 
switch to 64bit userland,
I just want to have the ability to compile some of my programs as 64bit.

regards,
chris