arm64: Add build salt to the vDSO

The vDSO needs to have a unique build id in a similar manner
to the kernel and modules. Use the build salt macro.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>