kbuild: rust: move rust/target.json to scripts/

scripts/ is a better place to generate files used treewide.

With target.json moved to scripts/, you do not need to add target.json
to no-clean-files or MRPROPER_FILES.

'make clean' does not visit scripts/, but 'make mrproper' does.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Miguel Ojeda <ojeda@kernel.org>