gcore, handle exited threads better

An early (and since discarded) version of this series tried to make
exited threads have distinct PTID between each other, and that change
exposed a problem in linux-tdep.c...  This was exposed by the
gdb.threads/gcore-stale-thread.exp testcase, which is exactly about
calling gcore with an exited thread selected:

 (gdb) [Thread 0x7ffff7fb6740 (LWP 31523) exited]
 PASS: gdb.threads/gcore-stale-thread.exp: continue to breakpoint: break-here
 gcore /home/pedro/gdb/binutils-gdb/build/gdb/testsuite/outputs/gdb.threads/gcore-stale-thread/gcore-stale-thread.core
 /home/pedro/gdb/binutils-gdb/build/../src/gdb/inferior.c:66: internal-error: void set_current_inferior(inferior*): Assertion `inf != NULL' failed.
 A problem internal to GDB has been detected,

That was find_inferior_ptid being called on the "exited" ptid, which
on that previous (and discarded attempt) had pid==-1.  The problem is
that linux-tdep.c, where it looks for the signalled thread, isn't
considering exited threads.  Also, while at it, that code isn't
considering multi-target either, since it is using
iterate_over_threads which iterates over all threads of all targets.
Fixed by switching to range-for iteration instead.

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* linux-tdep.c (find_signalled_thread(thread_info *,void *)):
	Delete.
	(find_signalled_thread()): New, factored out from
	linux_make_corefile_notes and adjusted to handle exited threads.
	(linux_make_corefile_notes): Adjust to use the new
	find_signalled_thread.

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