portage: Add support for --changed-use

The `--changed-use` flag is an improvement over `--newuse` because it
does not trigger rebuilds for USE flag changes that would not affect the
installed package. Its use is generally recommended over `--newuse`.

Signed-off-by: Dustin C. Hatch <dustin@hatch.name>
Merge pull request #8775 from AdmiralNemo/portage-changed-use

portage: Add support for --changed-use