Fix gdb.multi/multi-arch-exec.exp blocking under high load/slow gdb

Fix gdb.multi/multi-arch-exec.exp blocking under high load/slow gdb

When running multi-arch-exec.exp under valgrind, the test succeeds
when the machine is not loaded, but blocks when the machine is highly
loaded (e.g. when running the testsuite with valgrind with -j X
where X is one more than the nr of available cores).

The problem is that the hello program dies too early due to the alarm (30).

So, increase the alarm timer.
Note that this does not make the test take longer (it takes about
3.5 seconds on my system).  As I understand, the alarm is just there
to avoid hello staying there forever in case of another problem.

2019-03-28  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.multi/hello.c (main): Increase alarm timer.