build: Add `--target' option.

* guix/scripts/build.scm (derivations-from-package-expressions): Add
  `package-derivation' parameter.
  (show-help, %options): Add `--target'.
  (guix-build): Use `package-cross-derivation' when `--target' is
  passed.
* tests/guix-build.sh: Add dry-run test with `--target'.
* doc/guix.texi (Invoking guix build): Document `--target'.