x86: keep legacy generated vdso files around in .gitignore file

Commit 93d73005bff4 ("x86/entry/vdso: Rename vdso_image_* to
vdso*_image") updated the vdso .gitignore file with the new filenames,
which is certainly not incorrect.

However, while adding new generated names is obviously the right thing
to do, you should *not* immediately remove the old filenames from the
.gitignore file when things move around or get renamed, because people
still have those old generated files in their build trees - and they
haven't suddenly become valid files to commit to the repository just
because they were moved or renamed.

While it's mostly just a slight visual nuisance for 'git status' that
can be fixed up with a clean build tree, it can become more serious than
that: see for example commit 04a3389b3535 ("Remove stale generated
'genheaders' file").

That commit removed up a stale generated file that had been carelessly
committed by a kernel developer because it wasn't properly ignored any
more and thus showed up as a new file in their tree.

Fixes: 93d73005bff4 ("x86/entry/vdso: Rename vdso_image_* to vdso*_image")
Cc: Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>