[hppa] Fix incorrect stack frame usage in vfork.

[hppa] Fix incorrect stack frame usage in vfork.

In a multithreaded environment the call to vfork was
incorrectly creating a stack frame. Given that the
child unwinds the stack frame first to call exec, it
will corrupt any values the parent stored in the stack
frame. The solution is to avoid creating a stack frame
unless required to call an error function, in which case
we are assured no child was created.