Manually save and restore stack pointer between ucontext switching.

[albertnetymk]

No description provided.

[EliasC]

The quicksort program from issue #179 still segfaults for me. Is fixing this issue the purpose of this PR?

[EliasC]

Addendum: With this PR, I cannot reproduce the segfault when debugging!

bash-3.2$ ./quicksort
Debug: Random list generated successfully
Segmentation fault: 11
bash-3.2$ lldb quicksort
(lldb) target create "quicksort"
Current executable set to 'quicksort' (x86_64).
(lldb) r
Process 10983 launched: '/Users/Elias/Dropbox/PhD/encore/programs/quicksort' (x86_64)
Debug: Random list generated successfully
Checking...
List was sorted correctly!
Process 10983 exited with status = 0 (0x00000000) 

[albertnetymk]

Yes, exactly. That' odd; it was working when I tested it on my air.

[EliasC]

Can we have a third opinion? :)

[albertnetymk]

Still, it works OK on my air. Anyone else could test it on mac?

[EliasC]

It seems this was one of those bugs where we would see crashes when run from within Emacs, but not otherwise. I will merge this.