Catch exception in lib/gdbserver-support.exp:gdb_exit

Catch exception in lib/gdbserver-support.exp:gdb_exit

I see the error when I run gdb-sigterm.exp with native-gdbserver
on x86_64-linux.

infrun: prepare_to_wait^M
Cannot execute this command while the target is running.^M
Use the "interrupt" command to stop the target^M
and then try again.^M
gdb.base/gdb-sigterm.exp: expect eof #0: got eof
gdb.base/gdb-sigterm.exp: expect eof #0: stepped 12 times
ERROR OCCURED: : spawn id exp8 not open
    while executing
"expect {
-i exp8 -timeout 10
            -re "$gdb_prompt $" {
                exp_continue
            }
            -i "$server_spawn_id" eof {
                wait -i $expect_out(spawn_id)
                unse..."
    ("uplevel" body line 1)
    invoked from within

In gdb-sigterm.exp, SIGTERM is sent to GDB and it exits.  However,
Dejagnu or tcl doesn't know this.

This patch is to catch the exception, but error messages are still
shown in the console and gdb.log.  In order to avoid this, we also
replace gdb_expect with expect.

gdb/testsuite:

2015-04-13  Yao Qi  <yao.qi@linaro.org>

	* lib/gdbserver-support.exp (gdb_exit): Catch exception
	and use expect instead of gdb_expect.