arm: vdso: Enable arm to use common headers

Enable arm 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>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://lkml.kernel.org/r/20200320145351.32292-25-vincenzo.frascino@arm.com