Cache .gnu_debugdata BFD

While looking at the output of "maint info bfd" with multiple
inferiors, I noticed that there were duplicate entries for
.gnu_debugdata.

There is no reason to re-create this BFD each time it is needed.  This
patch arranges to share the data.

gdb/ChangeLog
2020-02-14  Tom Tromey  <tom@tromey.com>

	* minidebug.c (gnu_debug_key): New global.
	(find_separate_debug_file_in_section): Use it.

Change-Id: If139f89f0f07db33f399afdbcfbf5aaeffe4de46