[llvm-readobj] Output valid JSON for GroupSections

The current implementation output the LLVM formatted heading for group
sections, which was not valid JSON. This patch provides two small
customization points that allow the heading to vary between the two
implementations, and another that allows the section members to be
output as valid JSON objects.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D137095