[gdb/testsuite] Fix gdb.base/batch-exit-status.exp with native-gdbserver

[gdb/testsuite] Fix gdb.base/batch-exit-status.exp with native-gdbserver

When running test-case gdb.base/batch-exit-status.exp with target board
native-gdbserver, I run into (added missing double quotes for clarity):
...
builtin_spawn $build/gdb/testsuite/../../gdb/gdb -nw -nx \
  -data-directory $build/gdb/testsuite/../data-directory \
  -iex "set height 0" -iex "set width 0" \
  -ex "set auto-connect-native-target off" \
  -iex "set sysroot" -batch ""^M
: No such file or directory.^M
PASS: gdb.base/batch-exit-status.exp: 1x: \
  No such file or directory: [lindex $result 2] == 0
FAIL: gdb.base/batch-exit-status.exp: 1x: \
  No such file or directory: [lindex $result 3] == $expect_status
...

As in commit a02a90c114c "[gdb/testsuite] Set sysroot earlier in
local-board.exp", the problem is the use of -ex for
"set auto-connect-native-target off", which makes that the last command to
be executed, and consequently determines the return status.

Fix this by using -iex instead.

Tested on x86_64-linux.