Check whether the big lock is held before invoking pre_thread_ll_create.

Check whether the big lock is held before invoking pre_thread_ll_create.

If the pre_thread_ll_create tracking function would be invoked without the
big lock being held, that would trigger a race condition in the tools that
implement this tracking function.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12458 a5019735-40e9-0310-863c-91ae7b9d1cf9