BACKPORT: x86/vdso: Enable x86 to use common headers

Enable x86 to use only the common headers in the implementation
of the vDSO library.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200320145351.32292-24-vincenzo.frascino@arm.com


(cherry picked from commit abc22418db02b986fc5623c035507b6357e191ed)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I05311771de9a6cf52a5b98812ac9ae0d933ac84a