x86/um: fix vDSO installation

x86/um: fix vDSO installation

The generic vDSO installation logic used by 'make vdso_install' requires
that $(vdso-install-y) is defined by the top-level architecture Makefile
and that it contains a path relative to the root of the tree.
For UML neither of these is satisfied.

Move the definition of $(vdso-install-y) to a place which is included by
the arch/um/Makefile and use the full relative path.

Fixes: f1c2bb8b9964 ("um: implement a x86_64 vDSO")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20260318-um-vdso-install-v1-1-26a4ca5c4210@weissschuh.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>