Also remove the SIGSEGV special case, which was probably because
hand-written __exit_with_stack_teardown stubs used to try to cause
SIGSEGV if the exit system call returned (which it never does, so
that dead code disappeared).
Also move the sigprocmask into the only case where it's necessary ---
the one where we unmap the stack that would be used by a signal
handler.
Change-Id: Ie40d20c1ae2f5e7125131b6b492cba7a2c6d08e9