Building gcc-4.0 source package under i386

[csj <csj@zapo.net>]

Could a developer subscribed to the list explain the following
anomaly to me? I had always assumed that amd64 was a separate
architecture. With the amd64 libs installed, the gcc-4.0 source
package fails to build on my machine.

alpha:~# uname -m
i686
alpha:~# apt-get build-dep gcc-4.0
Reading Package Lists... Done
Building Dependency Tree... Done
Package libc6-dev-amd64 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
The following NEW packages will be installed:
  amd64-libs amd64-libs-dev lib64gcc1
0 upgraded, 3 newly installed, 0 to remove and 2 not upgraded.
E: Package libc6-dev-amd64 has no installation candidate
E: Failed to process build dependencies