klcc-cross: force rebuild for each MACHINE of the same arch

In case of subsequent builds for machines belonging to the same arch
we have to rebuild the klcc-cross wrapper because it is harcoding
the path to the headers in machine sysroot.
This hack is necessary to avoid to mark the klibc infrastructure as
machine-specific.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>