Add the gui & native OpenROAD platforms

[mithro]

The Google internal BUILD rules for OpenROAD support multiple values for platform;

string_flag(
    name = "platform",
    build_setting_default = "cli",
    values = [
        "cli",
        "headless",
        "native",
    ],
)

Currently the headless and native where not exported due to complications around getting qt5 into the bazel build system. That should be fixed.

[mithro]

The repository at https://github.com/justbuchanan/bazel_rules_qt seems to have suitable bazel qt rules.

[mithro]

Being able to access the GUI with a command like the following is pretty useful;

# For interactive use with GUI functionality (glinux only)
# blaze run --//third_party/open_road:platform=native //third_party/open_road:openroad -- -gui
config_setting(
    name = "platform_native",
    flag_values = {
        ":platform": "native",
    },
)