pahole: Factor out routine to process "--btf_features=default"
As we'll use it to process "--btf_features=+reproducible_build" meaning
"default + reproducible_build", as a more compact way to express that,
one that follows a command line syntax also present in 'perf report
--fields".
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/lkml/20240419205747.1102933-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>