clang: Set -DLLVM_APPEND_VC_REV=OFF

[kraj]

Fixes Issue #991

If this is not disabled, the build tries to use CMake magic to compute repository and sha1 its building, however in case of OE it gets it completely wrong when building from tarball, because its building under TMPDIR which maybe a directory under main repo checkout e.g. poky tree so it traverses up and finds the sha of poky and assumes that its building from a git tree instead of tarball and emits the version control info into clang -v output e.g.

❯ ../image/mnt/b/yoe/master/build/tmp/work/x86_64-linux/clang-native/20.0.0/recipe-sysroot-native/usr/bin/clang -v clang version 20.0.0 (/home/kraj/work/llvm-project f133c59b33a749dc6d5fa8fb9a2366b95ea45068)

This can contain buildpaths since work-shared is where llvm sources will be and they will appear here.

Therefore, avoid cmake trying to do this and we get good clean version info

../recipe-sysroot-native/usr/bin/aarch64-yoe-linux/aarch64-yoe-linux-clang
clang version 20.0.0

This will also avoid emitting buildpaths into debuginfo in some cases where compiler version is emitted into build.id info

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