server/gdb: fix return of gdb remote monitor command

Current implementation for gdb remote monitor command uses the
command_run_line() to execute the command.
While command_run_line() has several advantages, it unfortunately
hides the error codes and outputs the result of the command
through LOG_USER(), which is not what gdb requires. See 'qRcmd' in
https://sourceware.org/gdb/onlinedocs/gdb/General-Query-Packets.html



Replace command_run_line() with Jim_EvalObj() and parse the output
to provide the proper result to gdb.

Can be tested by defining in OpenOCD:
	proc a {} {return hello}
	proc b {} {return -code 4}
	proc c {} {return -code 4 "This is an error!"}
then by executing in gdb console:
	monitor a
	monitor b
	monitor c
	monitor foo

Change-Id: I1b85554d59221560e97861a499e16764e70c1172
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Torbjorn Svensson <torbjorn.svensson@st.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6886
Tested-by: jenkins