layer.conf: disable conflicting LLVM PACKAGECONFIG options

[lumag]

The LLVM package in OE-Core got optional support for building libclc and SPIRV-LLVM-Translator. However this also means that llvm-native also install clang binaries into the sysroot. In order to remove a conflict with the clang recipe from meta-clang forcibly disable PACKAGECONFIG options causing conflict.

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