[spu] Fix various test cases

[spu] Fix various test cases

The SPU-specific test cases were not modified to use standard_output_file
and therefore all were no longer being executed.  Fixing this exposed a
few other bugs in spu-info noticed by using a more recent compiler, which
are also fixed here.

gdb/testsuite/ChangeLog:
2017-11-26  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.arch/spu-info.c: Include <unistd.h>.
	(do_signal_test): Fix broken calls to write.
	* gdb.arch/spu-info.exp: Use prepare_for_testing.
	Fix checks for empty mailboxes.  Update signal tests for corrected
	do_signal_test routine.  Allow nonzero event status.