Support kernel compilation with Clang/LLVM

[Randl]

Description

The kernel supports compilation with LLVM in addition to GCC, including cross-compilation to arm and riscv. This PR adds an option to use KERNEL_COMPILER=clang to compile kernel with clang.

Documentation summary for feature / change

I haven't added documentation but probably should?

How Has This Been Tested?

• Built armbian with clang for one target of each architecture (x86, arm, riscv)
• Verified that gcc build still works

Checklist:

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings

[igorpecovnik]

I haven't added documentation but probably should?

That would be helpful. We need to improve our documentation. Upper part is good while below is chaos https://docs.armbian.com/Developer-Guide_Build-Options/#build-options-below-needs-to-be-retested-and-added-above-could-be-deprecated and I hope someone finds this ticket ;)

[Randl]

So if I understand correctly, this should be a separate PR to the https://github.com/armbian/documentation/ repo?

[adeepn]

So if I understand correctly, this should be a separate PR to the https://github.com/armbian/documentation/ repo?

Yes