s390/vdso: remove unused ENTRY in linker scripts

When linking vdso64.so.dbg with ld.lld, there is a warning about not
finding _start for the starting address:

  ld.lld: warning: cannot find entry symbol _start; not setting start address

Fix this by removing the unused ENTRY in both vdso linker scripts. See
commit e2471728

 ("powerpc/vdso: Remove unused ENTRY in linker
scripts"), which solved the same problem for powerpc, for further details.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>