the ".rc" extension is not really standardized and these files correspond to very different things.
For example:
android/system/core/rootdir/init.rc
is the top-level /init.rc that will end up being executed by the 'init' process at boot
android/system/core/rootdor/etc/init.goldfish.rc
is the emulator-specific instructions for 'init' that are only run in the emulator
android/external/qemu/images/android_icon.rc
is a Windows "resource" file used to embed the Android icon into the final Windows executable when generating the emulator.
other .rc files are probably "resource containers" or stuff like that that were inherited by various external projects and probably not used in Android at all. You will have to look at the modules where they are used to be sure.