Follow up to r14313: disable stack cache earlier
glibc is recycling memory for detached threads before a thread
termination => disable the stack cache earlier (i.e. once
a 'non main thread' is seen)



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14329 a5019735-40e9-0310-863c-91ae7b9d1cf9