stubdom: disable newlib's need for texinfo

stubdom: disable newlib's need for texinfo

newlib has a hardcoded dependency on texinfo, which in turn means that
Xen has this dependency when building the stub domains. The files that
newlib generates are never used or exposed by the Xen build system so
its really a wasted effort to require a heavy (size wise) package for
the build of something that's thrown away. Instead of adding it as a
requirement this patches out the code path in newlib that uses it so
that the files are never generated.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>