[hppa] Fix *context stack usage for varargs

[hppa] Fix *context stack usage for varargs

The getcontext, and setcontext functions should
not adjust the stack, and should load the stack
pointer from the machine context. Calling makecontext
should create a frame for spilled incoming arguments,
and sync uc_stack.ss_sp to the machine context.
We do not support calling getcontext, modifying ss_sp,
and calling setcontext directly.