Match shell_prompt # in batch-preserve-term-settings.exp

Match shell_prompt # in batch-preserve-term-settings.exp

batch-preserve-term-settings.exp fails if the shell prompt isn't $.  It
is # in our testing env.  In fact, the shell prompt can be anything.

The perfect solution would be "set_board_info shell_prompt" in the
host board file, and use board_info shell_prompt in
batch-preserve-term-settings.exp.  This is a little bit overkill to
me, and we still need to figure out the different prompts on different
shells.  I also tried to start shell with the prompt preset, but there is
not unique way to set shell prompt in different shells, so I give up.

It is reasonably simple to match either $ or # for the shell prompt, and
we can easily extend it to match other char, like >.

gdb/testsuite:

2016-05-16  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/batch-preserve-term-settings.exp: Remove variable
	shell_prompt.  Update shell_prompt_re.