Fix 372504 Hanging on exit_group

Fix 372504  Hanging on exit_group

Note that it is unclear if the PRE syscall for rt_sigsuspend
is properly setting up a temporary mask in the thread state
tmp_sig_mask:  if an handler is called while a thread is
calling sigsuspend, the mask during the handler run must be
the temporary mask set by sigsuspend.
It is not clear if/where the valgrind sigframe builder/handler
sets the tmp_sig_mask to the value as expected by the user
(i.e. the value of the temporary mask which was given to
the sigsuspend syscall)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16141