Na documentação do Fedora para rpm, arquivos spec e rpmbuild:
The --target option sets the target architecture at build time. Chapter 3,
Using RPM covers how you can use the --ignoreos and --ignorearch options
when installing RPMs to ignore the operating system and architecture that
is flagged within the RPM. Of course, this works only if you are installing
on a compatible architecture.
On the surface level, the --target option overrides some of the macros in
the spec file, %_target, %_target_arch, and %_target_os. This flags the RPM
for the new target platform.
Under the covers, setting the architecture macros is not enough. You really
cannot create a PowerPC executable, for example, on an Intel-architecture
machine, unless you have a PowerPC cross compiler, a compiler that can make
PowerPC executables.
http://docs.fedoraproject.org/pt-BR/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch-rpmbuild.html
Portanto, como ele diz, verifique se você possui os compiladores adicionais instalados (por exemplo, gcc.i686 e gcc.x86_64).