x86/entry/vdso: Rename vdso_image_* to vdso*_image

x86/entry/vdso: Rename vdso_image_* to vdso*_image

The vdso .so files are named vdso*.so. These structures are binary
images and descriptions of these files, so it is more consistent for
them to have a naming that more directly mirrors the filenames.

It is also very slightly more compact (by one character...) and
simplifies the Makefile just a little bit.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251216212606.1325678-2-hpa@zytor.com