Have the inner Valgrind registering the interim_stack asap.

Have the inner Valgrind registering the interim_stack asap.
This is needed to have the outer valgrind able to do stack
traces of the inner Valgrind before the main thread runs.
Without this, the outer crashes (segv) when doing a stack trace.
In mtV.txt, an ugly kludge was described to avoid this crash.
This is the clean solution replacing the kludge.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12638