gdbserver x86 on win32: call init_target_desc

gdbserver x86 on win32: call init_target_desc

When trying to run gdbserver compiled for x86 win32 under wine, I get:

  $ wine ./gdbserver/gdbserver.exe --once :1234 ./test
  /home/emaisin/src/binutils-gdb/gdb/gdbserver/regcache.c:177: A problem internal to GDBserver has been detected.
  regcache* new_register_cache(const target_desc*): Assertion `tdesc->registers_size != 0' failed.

It seems like on that platform, init_target_desc is never called, so
registers_size is never computed.

My first thought was to call init_target_desc somewhere in win32-low.c,
but it turns out that when using win32 on arm, the target description is
already initialized by the generated code.  My second thought was to
call it in {i386,amd64}_create_target_description, but those functions
are shared with GDB, and init_target_desc is gdbserver-specific.  So I
ended up with the simplest fix, calling it in i386_arch_setup.

Now I hit some other problem:

  $ wine ./gdbserver/gdbserver.exe --once :1234 ./test
  Killing process(es): 39
  No program to debug
  Exiting

but still, I think fixing the tdesc issue this is a step forward.

gdb/gdbserver/ChangeLog:

	* win32-i386-low.c (i386_arch_setup): Call init_target_desc.