www-client/chromium: wire-up cross-compile support

www-client/chromium: wire-up cross-compile support

Cross-compiling still does not work, but this gets us closer.

- Build GN with the host (CBUILD) compiler, skip rebuild after bootstrap.
- Add a toolchain definition for the host (CBUILD) compiler.
- Set the host_toolchain option.
- Set the target_cpu option based on the target arch (CHOST).

The host_toolchain option is also set for native builds. This makes
clang work without setting clang_base_path.

Package-Manager: Portage-2.3.6_p7, Repoman-2.3.2_p75