gdb: move setbuf calls out of gdb_readline_no_editing_callback

After this commit:

  commit d08cbc5d
  Date:   Wed Dec 22 12:57:44 2021 +0000

      gdb: unbuffer all input streams when not using readline

Issues were reported with some MS-Windows hosts, see the thread
starting here:

  https://sourceware.org/pipermail/gdb-patches/2022-March/187004.html

Filed in bugzilla as: PR mi/29002

The problem seems to be that calling setbuf on terminal file handles
is not always acceptable, see this mail for more details:

  https://sourceware.org/pipermail/gdb-patches/2022-April/187310.html

This commit does two things, first moving the setbuf calls out of
gdb_readline_no_editing_callback so that we don't end up calling
setbuf so often.

Then, for MS-Windows hosts, we don't call setbuf for terminals, this
appears to resolve the issues that have been reported.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29002