buildroot using external toolchain – rootfilesystem creating

I am trying to build my own rootfile system for RPI using buildroot.
I am new to buildroot.

I have downloaded following cross compiler folder from git :–
as mentioned in this link :–

How i will come to know what is the kernel header is used by above cross compiler ?

My installer GCC compiler is :–

crosscompiler is downloaded from git at following location:–

Please suggest what value to fill here for toolchain selection :–

Toolchain type : External toolchain
Toolchain : Custom toolchain
Toolchain origin : Pre-installed toolchain
Toolchain path : ?
Toolchain prefix : ?
external Toolchain kernel header series : ?
external toolchain c library : ?