x86/percpu, xen: Correct PER_CPU_VAR() usage to include symbol and its addend

The PER_CPU_VAR() macro should be applied to a symbol and its addend.
Inconsistent usage is currently harmless, but needs to be corrected
before %rip-relative addressing is introduced to the PER_CPU_VAR() macro.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: linux-kernel@vger.kernel.org
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sean Christopherson <seanjc@google.com>