@doniks Thanks for the post, it gave me clues on my issue.
This is an old thread but wanted to add to your post because it took me so long to get it to compile. Having never compiled an Android Kernel before I think I spent 2 days on it.
Using Ubuntu 18, gcc-4.8 was not available through apt. I ended up download the AOSP8 source. The source contains gcc for the android version located in prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android
Looking at the build.config.aarch64 file https://android.googlesource.com/kernel/common/+/a5d10f0defc377d43672b304e31e37a517fbf0bf/build.config.aarch64
I found the variables, needed for the kernel source.
There were a lot of other dependencies I had to install but that took care of the correct gcc version.
Hope this helps someone.