x86/boot/build: Add cpustr.h to targets and remove clean-files

Files in $(targets) are always cleaned up. Move the 'targets' assignment
out of the ifdef and remove 'clean-files'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200215063241.7437-1-masahiroy@kernel.org