xen/control: gate x86 specific code in the preprocessor

Commit 152265223048 was implemented strictly for x86. Unfortunately
one of the pieces was mixed into a common area breaking other
architectures. For now disable these bits on !x86, this should be
cleaned up later.

Fixes: 152265223048 ('xen: fix dropping bitmap IPIs during resume')
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29306