[minicoredumper] Handling of unavailable bytes in minicoredumps by GDB

[John Ogness]

Hi,

A modified version of binutils (bfd/gdb) to automatically respect the
minicoredumper dump list is available here:

    https://github.com/Linutronix/binutils-gdb/tree/minicoredumper-section-note

This is based on the previous work of Simon Marchi. The modifications to
bfd result in elf_core_file_p() and elf_object_p() being nearly
identical. It makes sense to unify these two functions if these changes
are to go upstream. It is also up for discussion if all elf sections
should be made available to bfd_core or only the new minicoredumper
section.

John Ogness