s390/vdso: Create .build-id links for unstripped vdso files

Citing Andy Lutomirski from commit dda1e95c ("x86/vdso: Create
.build-id links for unstripped vdso files"):

"With this change, doing 'make vdso_install' and telling gdb:

set debug-file-directory /lib/modules/KVER/vdso

will enable vdso debugging with symbols.  This is useful for
testing, but kernel RPM builds will probably want to manually delete
these symlinks or otherwise do something sensible when they strip
the vdso/*.so files."

Fixes: 4bff8cb5 ("s390: convert to GENERIC_VDSO")
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>