Revert "Remove gcc, clang and python prebuilts"

This reverts commit 78fe28c0eb30e471e2daa0931f050719df942049.

Signed-off-by: Constantin Runge <uroc327@cssbook.de>
Move prebuilts under the other manifest stuff and add manifest and repo to manifest group

Signed-off-by: Constantin Runge <uroc327@cssbook.de>