pahole: Allow asking for extra features using the '+' prefix in --btf_features

pahole: Allow asking for extra features using the '+' prefix in --btf_features

Instead of the more verbose form:

  --btf_features=default,reproducible_build

That means "the set of default BTF features plus the 'reproducible_build'
extra BTF feature", use + directly, making it more compact:

  --btf_features=+reproducible_build

In the future we may want the '-' counterpart as a way to _remove_ some
of the default set of BTF features.

Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Xu <dxu@dxuuu.xyz>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/dwarves/20240419205747.1102933-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>