When identifying stacks so as to decide whether or not to make a self

When identifying stacks so as to decide whether or not to make a self
checking translation in the case --smc-check=stack (the default),
don't use SF_GROWDOWN as an indication of stackness, since that only
is set for the main stack.  Instead establish whether code is being
taken from a stack by seeing if the requesting thread's stack pointer
points into the same area that the translation is being taken from.

This makes trampolining work even for threaded programs.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@4303