Fix regression revealed by corethreads.exp

Fix regression revealed by corethreads.exp

This patch fixes a regression introduced by:
https://sourceware.org/ml/gdb-patches/2015-12/msg00192.html

We can't use thread_from_lwp with core files.  As mentioned in a comment,
td_ta_map_lwp2thr uses ps_get_thread_area, but we can't use that
currently on core targets, as it uses ptrace directly.

Use directly record_thread instead.

This fixes :
PASS -> FAIL: gdb.threads/corethreads.exp: thread0 found
PASS -> FAIL: gdb.threads/corethreads.exp: thread1 found

gdb/ChangeLog:

	* linux-thread-db.c (find_new_threads_callback): Use record_thread.