package/xen: disable go tools

package/xen: disable go tools

go tools are enabled by default since version 4.14.0 and
https://github.com/xen-project/xen/commit/1e13ea98bbb8369d065af4666c45c26dab5240d5

Disable them to avoid the following build failure on arm or aarch64
(when host-go is built before xen):

aarch64-none-linux-gnu-gcc: error: unrecognized command line option '-m64'

Fixes:
 - http://autobuild.buildroot.org/results/08948e4c4a88fbf09cae524949e5b656882a6e9a
 - http://autobuild.buildroot.org/results/d966585c931ca4a4ae6a854a21e07eec4c277a02

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>