Increase the stack size soft limit to 32MiB

See <https://bugzilla.wikimedia.org/show_bug.cgi?id=71486#c6>.
HHVM's default thread count is CPU cores * 2, so 48 for most app servers. This
means a cumulative max size of 1.3 GiB for all threads, which is acceptable.
The hard limit stays unlimited, as before.

Change-Id: I455c0e9f1746ba9f79c0e7c58d481f1677879783