Check the right proc name

Check the right proc name

In lib/gdbserver-support.exp, we rename gdb_exit to
gdbserver_orig_gdb_exit, but we check the existence gdbserver_gdb_exit.
We should check gdbserver_orig_gdb_exit instead.  Looks it is a typo
or an oversight.

gdb/testsuite:

2016-09-22  Yao Qi  <yao.qi@linaro.org>

	* lib/gdbserver-support.exp: Check the existence of
	gdbserver_orig_gdb_exit rather than gdbserver_gdb_exit.