[ELF] Add --print-archive-stats=

gold has an option --print-symbol-counts= which prints:

  // For each archive
  archive $archive $members $fetched_members
  // For each object file
  symbols $object $defined_symbols $used_defined_symbols

In most cases, `$defined_symbols = $used_defined_symbols` unless weak
symbols are present. Strangely `$used_defined_symbols` includes symbols defined relative to --gc-sections discarded sections.
The `symbols` lines do not appear to be useful.

`archive` lines are useful: `$fetched_members=0` lines correspond to
unused archives. The information can be used to trim dependencies.

This patch implements --print-archive-stats= which prints the number of
members and the number of fetched members for each archive.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D78983