x86: files: Make sys/x86/x86/pvclock.c optional

Make inclusion of sys/x86/x86/pvclock.c contingent on that of its
dependents.

Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31417

(cherry picked from commit 346f5a0c483b7c159efd7c8faf4a18f3e3f77158)