Add aconfig flags and a generic generated library plugin module for cc

The generated module lets us keep the aconfig code in its own pacakge
and not infect all of the cc package with aconfig. It's also closer
to what bazel is going to do

Bug: 283479529
Test: m aconfig_hello_world_cc && adb push $TOP/out/target/product/panther/system/bin/aconfig_hello_world_cc /system/bin && adb shell aconfig_hello_world_cc
Change-Id: I2fb9e419939c7ca77b111da9c376af077e2348a9