bpftool, selftests/hid/bpf: Fix 29 clang warnings

When building either tools/bpf/bpftool, or tools/testing/selftests/hid,
(the same Makefile is used for these), clang generates many instances of
the following:

    "clang: warning: -lLLVM-17: 'linker' input unused"

Quentin points out that the LLVM version is only required in $(LIBS),
not in $(CFLAGS), so the fix is to remove it from CFLAGS.

Suggested-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20240505230054.13813-1-jhubbard@nvidia.com