Fix signals-state-child.exp in remote testing

Fix signals-state-child.exp in remote testing

Remote testing isn't considered in signals-state-child.exp, so the it
fails like

shell diff -s /scratch/yao/gdb/build-git/aarch64-linux-gnu/gdb/testsuite/outputs/gdb.base/signals-state-child/standalone.txt /scratch/yao/gdb/build-git/aarch64-linux-gnu/gdb/testsuite/outputs/gdb.base/signals-state-child/gdb.txt^M
diff: /scratch/yao/gdb/build-git/aarch64-linux-gnu/gdb/testsuite/outputs/gdb.base/signals-state-child/standalone.txt: No such file or directory^M
(gdb) FAIL: gdb.base/signals-state-child.exp: signals states are identical

This patch is to fix it.

gdb/testsuite:

2016-08-23  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/signals-state-child.exp: Set variables gdb_txt and
	standalone_txt.  Delete gdb_txt and standalone_txt on host
	and target.  Spawn the binary on target.  Copy files from
	target to host.