bug#33315: gdb set remote exec-file error.

[Asif <asif.haswarey@gmail.com>]

Summary:
Trying to set a remote executable file name in the gdb *gud* buffer
causes the a single space to be appended to the executable file name.

Steps to reproduce the error:
1) Start emacs.
2) Enable gdmi debug messages in *Messages* buffer.
   M-: (setq gdbmi-debug-mode t)
3) Open the *Messages* buffer and go to *scratch* buffer.
   C-x 2
   C-x b *Messages*
   M-x next-multiframe-window
4) Start gdb which creates the *gud* buffer and makes it the current buffer.
   M-x gdb
   Run gdb (like this): gdb -i=mi
5) Enter the gdb remote exec-file name in the *gud* buffer.
   M-x b *gud*
   set remote exec-file xxx
6) The *Messages* buffer shows the wrong exec-file name value "xxx ",
with the appended space, instead of the correct value "xxx". When this
is sent to the remote gdbserver, the gdbserver report error saying
that remote exec file "xxx " does not exist.
7) *Message* contents shows the following:
   ...
   gdbmi-bnf-incomplete-record-result: param="remote exec-file",value="xxx "
   ...
8) The incorrect exec-file name string is passed to the function named
"gud-filter" located in:
emacs-26.1/lisp/progmodes/gud.el
_
Asif