x86: address violations of MISRA C Rule 8.4

x86: address violations of MISRA C Rule 8.4

Rule 8.4 states: "A compatible declaration shall be visible when an
object or function with external linkage is defined."

These variables are only referenced from assembly code, so they need to
be extern and there is negligible risk of them being used improperly
without noticing.

As a result, they can be exempted using a comment-based deviation.
No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>