arm: Add support for generic vDSO

The arm vDSO library requires some adaptations to use to take advantage
of the newly introduced generic vDSO library.

This patch introduces the following changes:
 - Modification vdso.c to be compliant with the common vdso datapage
 - Use of lib/vdso for gettimeofday
 - Implementation of __vdso_time
 - Implementation of elf note

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>