supertuxkart: Constrain to x86 architecture

Our build system has detected that angelscript can't be
cross-compiled properly. This has been also reported to
supertuxkart [1], and although the developers were open
to take patches fixing STK embedded angelscript, it was
pointed out that non-x86 architectures were not officially
supported and of little interest [2].

Hence, let's constrain the package selection and the host
architecture to x86.

[1] https://github.com/supertuxkart/stk-code/issues/2569
[2] https://github.com/supertuxkart/stk-code/pull/2195

Fixes: http://autobuild.buildroot.net/results/655/655f046b3bb5f7a4ea492328b608e409d6769689/


Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>