Introduce infrastructure for cross-compiling Fedora kernels
Intended to assist building for secondary architectures like ppc64, s390x,
To use, create an .src.rpm using "fedpkg srpm" and then run
"rpmbuild --rebuild --with cross --without perf --without tools \
kernel*.src.rpm" to cross compile. This requires binutils and gcc
packages named like %_target_cpu, which all but powerpc64 currently provides
in rawhide/F-19. Can't (currently) cross compile perf or kernel-tools, since
libc is missing from the cross environment.