sal: create threads on Linux with a larger stack for ASAN

The default on Fedora 20 is 8MB, which causes stack overflows from
CPython code in some of the JunitTests when built with clang 3.4
-fsanitize=address.

Change-Id: I3de9975564aad668e746cea74ae10931ef36a608
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>