Properly set alarm value in gdb.threads/non-stop-fair-events.exp

Properly set alarm value in gdb.threads/non-stop-fair-events.exp

Nowadays, the alarm value is 60, and alarm is generated on some slow
boards.  This patch is to pass DejaGNU timeout value to the program,
and move the alarm call before going to infinite loop.  If any thread
has activities, the alarm is reset.

gdb/testsuite:

2015-04-07  Yao Qi  <yao.qi@linaro.org>

	* gdb.threads/non-stop-fair-events.c (SECONDS): New macro.
	(child_function): Call alarm.
	(main): Move call to alarm into the loop.
	* gdb.threads/non-stop-fair-events.exp: Build program with
	-DTIMEOUT=$timeout.