Add log summary to tombstones

When the tombstones are uploaded to APR, they're truncated at 64KB.
This causes the log data, which is at the end, to be lost if the
process has more than about 12 threads (which many do).

This change adds the last few lines of the log right below the
report for the crashing thread, where we should be guaranteed to
keep it.

Also, clean up trailing newlines on log messages (which end up in
the tombstone), and don't print a "------- log" banner if there
aren't any messages in that log file (e.g. slog).

Also also, don't try to show_nearby_maps unless this is the crashing
thread.

Bug 5471955

Change-Id: Iaa4fd2fafbaeda2f20bb95f202177d7744a91f9d