server/gdb: fix gdb remote monitor cmd on multi-target

Commit 5ebb1bde

 ("server/gdb: fix return of gdb remote monitor
command") replaces the call to command_run_line() with call to
Jim_EvalObj() but does not properly set the "context".
In multi-target environment, his can cause the erroneously
execution of the command on the wrong target.

Copy from the code in command_run_line() the proper setup before
executing Jim_EvalObj().

Change-Id: I56738c80779082ca146a06c01bc30e28bc835fd3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Bohdan Tymkiv <bohdan200@gmail.com>
Fixes: 5ebb1bde ("server/gdb: fix return of gdb remote monitor command")