Today, when gdbproxy is started with --no-start mode, GDB fails to
recognise the symbol file unless the `file` command or the
`sharedlibrary` command is explicitly issued.

Also, RHEL's downstream GDB complains with the following message:

  Remote gdbserver does not support determining executable automatically.
  RHEL <=6.8 and <=7.2 versions of gdbserver do not support such automatic executable detection.
  The following versions of gdbserver support it:
  - Upstream version of gdbserver (unsupported) 7.10 or later
  - Red Hat Developer Toolset (DTS) version of gdbserver from DTS 4.0 or later (only on x86_64)
  - RHEL-7.3 versions of gdbserver (on any architecture)

Fix this by implementing the qXfer object "exec-file".
Signed-off-by: Jinoh Kang <jinoh.kang.kr@gmail.com>
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>