[gdb/testsuite] Fix gdb_target_cmd

[gdb/testsuite] Fix gdb_target_cmd

In commit 81dc3ab594 "[gdb/testsuite] Handle unreachable network in
server-connect.exp" a regression was introduced in gdb_target_cmd, causing
ERRORs like this:
...
ERROR: tcl error sourcing src/gdb/testsuite/gdb.server/abspath.exp.
ERROR: wrong # args: should be "gdb_target_cmd {$args}"
    while executing
"gdb_target_cmd $gdbserver_protocol $gdbserver_gdbport"
...

Fix the argument passing in gdb_target_cmd.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdbserver-support.exp (gdb_target_cmd): Fix argument passing.