Fix leak by using td_ta_delete() to deregister target process and deallocate internal...

Fix leak by using td_ta_delete() to deregister target process and deallocate internal process handle.

Valgrind reports the below leak:

==25327== VALGRIND_GDB_ERROR_BEGIN
==25327== 672 bytes in 1 blocks are definitely lost in loss record 2,759 of 3,251
==25327==    at 0x4C2E07C: calloc (vg_replace_malloc.c:752)
==25327==    by 0x7FDCB3E: ???
==25327==    by 0x532A7A: try_thread_db_load_1 (linux-thread-db.c:828)
==25327==    by 0x532A7A: try_thread_db_load(char const*, int) (linux-thread-db.c:997)
==25327==    by 0x53354D: try_thread_db_load_from_sdir (linux-thread-db.c:1074)
==25327==    by 0x53354D: thread_db_load_search (linux-thread-db.c:1129)
==25327==    by 0x53354D: thread_db_load() (linux-thread-db.c:1187)
==25327==    by 0x611AF1: operator() (functional:2127)
==25327==    by 0x611AF1: notify (observable.h:106)
==25327==    by 0x611AF1: symbol_file_add_with_addrs(bfd*, char const*, enum_flags<symfile_add_flag>, std::vector<other_sections, std::allocator<other_sections> >*, enum_flags<objfile_flag>, objfile*) (symfile.c:1158)
==25327==    by 0x5F5C4A: solib_read_symbols(so_list*, enum_flags<symfile_add_flag>) (solib.c:691)
==25327==    by 0x5F6A8B: solib_add(char const*, int, int) (solib.c:1003)
==25327==    by 0x5F6BF7: handle_solib_event() (solib.c:1281)
==25327==    by 0x3D0A94: bpstat_stop_status(address_space const*, unsigned long, thread_info*, target_waitstatus const*, bpstats*) (breakpoint.c:5417)
==25327==    by 0x4FF133: handle_signal_stop(execution_control_state*) (infrun.c:5874)
==25327==    by 0x502C29: handle_inferior_event_1 (infrun.c:5300)
==25327==    by 0x502C29: handle_inferior_event(execution_control_state*) (infrun.c:5335)
==25327==    by 0x5041DB: fetch_inferior_event(void*) (infrun.c:3868)
==25327==    by 0x4A1E7C: gdb_wait_for_event(int) (event-loop.c:859)
...

This leak is created because a call to td_ta_new allocates some resources
that must be freed with td_ta_delete, and that was missing.

With this patch, the nr of GDB executions leaking during regression tests
decreases further from 566 to 380.

Note that the gdbserver equivalent code is properly calling
td_ta_delete: see thread_db_mourn in thread-db.c.

Tests run natively on debian/amd64, and run under valgrind.

gdb/ChangeLog
2018-12-08  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* linux-thread-db.c (struct thread_db_info): Add td_ta_delete_p.
	(thread_db_err_str): Forward declare.
	(delete_thread_db_info): Call td_ta_delete_p if available.
	(try_thread_db_load_1): Acquire td_ta_delete address.
	* nat/gdb_thread_db.h (td_ta_delete_ftype): Declare.