Make linux native always have at least one thread

[Pedro Alves]

The non-stop mode requires gdb to always have a thread in
the thread list, so we can use the new non-stop MI options
on it.  In ST programs, the main thread should be listed as
the single thread in the thread list.  Currently, when
debugging a single-threaded application in native linux,
"info threads" comes out empty.

Ideally, every platform should do this, so we can get rid
of a lot of weird ugly code to work around that fact
(state is stored and swapped between plain old global
variables, and struct thread_info, struct execution_control_state,
and struct inferior_status depending on what gdb is doing).

Daniel recently made this fix very simple, by removing
the tid component of a thread_db thread off of the
ptid of each thread.

So, this patch closes the issue for linux native.

Tested on x86_64-unknown-linux-gnu.

OK?

-- 
Pedro Alves
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