selftests/bpf: Clarify llvm dependency with btf_tag selftest

btf_tag selftest needs certain llvm versions (>= llvm14).
Make it clear in the selftests README.rst file.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211112012651.1508549-1-yhs@fb.com