x86/vdso: Consolidate targets and clean-files

'targets' and 'clean-files' do not need to list the same files because
the files listed in 'targets' are cleaned up.

Refactor the code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231121235701.239606-2-masahiroy@kernel.org