llvm-config: update following the oe-core script

[lumag]

The Mesa Clover meson.build script passes two options to llvm-config script, --libs --ldflags. The script from meta-clang passes control to the native llvm-config script, which unfortunately results in the native dynamic linker option leaking to the cross build. Fix that by adopting the approach from OE-core and filter known options before calling into the native llvm-config.

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