This solves the question on my Ubuntu box. Ubuntu provides gcc-multilib packages. Here is what I installed on this box (most of them pulled in because of dependencies):
I was able to compile stock 2.6.18 kernel with this setup with the following command:
make ARCH=x86_64 bzImage
I did not need to anything to CROSS_COMPILE variable in the top Makefile.
Before compiling I run:
make ARCH=x86_64 menuconfig
and changing processor type to "Generic X86_64"
However, RedHat does not seem to provide gcc multilib packages and RHEL5 is our official build environment. I wonder if anybody knows how to do this trick on RHEL5.
Any help greatly appreciated.