Test --wrapper in extended-remote

Test --wrapper in extended-remote

We didn't test --wrapper option in extended-remote before, this patch
is to add a test case for it.  In order to pass option --wrapper to
gdbserver in extended-remote, I add arg in gdbserver_start_extended,
and its default value is "", so that other places use
gdbserver_start_extended don't have to be updated.

gdb/testsuite:

2015-07-24  Yao Qi  <yao.qi@linaro.org>

	* lib/gdbserver-support.exp (gdbserver_start_extended): Add
	argument options.
	* gdb.server/ext-wrapper.exp: New file.