Make gdb_stdout&co be per UI

Make gdb_stdout&co be per UI

We need to have these send output to the proper UI.

However, this patch still make them look like globals.  Kind of like
__thread variables, if you will.  Changing everything throughout to
write something like current_ui->gdb_stdout instead would be massive
overkill, IMNSHO.

This leaves gdb_stdtargin/stdtarg/stdtargerr global, but maybe that was a
mistake, I'm not sure -- IIRC, MI formats target I/O differently, so
if we have a separate MI channel, then I guess target output should go
there instead of to gdb's stdout.  OTOH, maybe GDB should send that
instead to "set inferior-tty", instead of multiplexing it over MI.  We
can always fix those later when it gets clearer where they should go.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* main.c (gdb_stdout, gdb_stderr, gdb_stdlog, gdb_stdin): Delete
	globals.
	(gen_ret_current_ui_field_ptr): New macro.  Use it to generate
	wrappers for gdb_stdout, gdb_stderr, gdb_stdlog and gdb_stdin.
	* top.h (struct ui) <m_gdb_stdout, m_gdb_stdin, m_gdb_stderr,
	m_gdb_stdlog>: New fields.
	(current_ui_gdb_stdout_ptr, current_ui_gdb_stdin_ptr)
	(current_ui_gdb_stderr_ptr, current_ui_gdb_stdlog_ptr): Declare.
	(gdb_stdout, gdb_stdin, gdb_stderr, gdb_stdlog): Reimplement as
	macros.