pmb.build.kconfig: Use LLVM for kconfig where applicable

Aelinrequested to merge
Aelin/pmbootstrap:kconfig-default-llvm into master

Assume that any kernel package with clang in makedepends wants to use LLVM for building.

This way we can use pmb kconfig migrate <package> to regenerate the kernel configs with LLVM if applicable and easily migrate the kernel packages over.

Edited by Aelin

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