Fix: 32-bit x86 strict-aliasing warnings

Compiling on x86-32 shows the following warnings for filter (with gcc
4.3 and 4.4):

././ust_tests_hello.h:28: note: initialized from here
././ust_tests_hello.h:28: error: dereferencing pointer ‘__stack_data.18’ does break strict-aliasing rules

Fix it by using memcpy when copying to the temporary "stack" array used
to send arguments to the filter.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>