Fix a regexp pattern in gdb.base/auto-connect-native-target.exp

Fix a regexp pattern in gdb.base/auto-connect-native-target.exp

When I test gdb head (for 7.8 release) on arm-none-eabi, I find the
following this failure, which are caused by the improper regexp
pattern in the test.

(gdb) help target native^M
Undefined target command: "native".  Try "help target".^M
(gdb) FAIL: gdb.base/auto-connect-native-target.exp: help target native

The space in front of "$gdb_prompt $" looks redundant, and this patch
is to remove it from the regexp pattern.

gdb/testsuite:

2014-06-04  Yao Qi  <yao@codesourcery.com>

	* gdb.base/auto-connect-native-target.exp: Remove redundant
	space from the regexp pattern.