Fix regexp pattern in gdb.base/default.exp

Fix regexp pattern in gdb.base/default.exp

I see this failure on arm-none-eabi gdb testing.

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

This patch is to update the regexp pattern to match "native" instead of
"child".

gdb/testsuite:

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

	* gdb.base/default.exp: Replace "child" with "native" in
	regexp pattern.