compat_build: use llvm-config from the llvm compat build
    
    The regular llvm build installs llvm-config-N to _bindir
    The compat_build llvm build install llvm-config-N to pkg_bindir
    Use the pkg_bindir one.
    
    Fixes a build error
    error: File not found: .../rpmbuild/BUILDROOT/
      clang13-13.0.0-5.fc36.x86_64/usr/lib64/llvm13/lib/*.so.*
    
    Signed-off-by: Tom Rix <trix@redhat.com>