Skip gdb.base/branch-to-self.exp if gdb,nosignals exists

Skip gdb.base/branch-to-self.exp if gdb,nosignals exists

I get a timeout fail in branch-to-self.exp when it is compiled by a
bare-mental target running qemu, which doesn't have signal.

The test should be skipped if gdb,nosignals exists, and that is
what this patch does.

gdb/testsuite:

2016-04-27  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/branch-to-self.exp: Skip it if gdb,nosignals
	exists.