-target has been deprecated since Clang 3.4 in 2013. Use the preferred
--target=bpf form instead. This matches how we use --target= in
scripts/Makefile.clang.
Link: https://github.com/llvm/llvm-project/commit/274b6f0c87a6a1798de0a68135afc7f95def6277
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>