Fix implementation on ppc64le

[npmccallum]

https://travis-ci.org/enarx/frenetic

At the above link you can see that tests fail for ppc64le. It is not currently known why. In order to solve this bug you need to:

1. Remove ppc64le from allow_failures.
2. Make all required tests pass.
3. Submit a pull request.

[haraldh]

I couldn't get this to work with all llvm magic. I could fix jump_init but not jump_swap. It just wouldn't swap or get a segfault. Also I saw writes to the stack beyond the stack top with valgrind, despite everything working on armv7l and amd64.

[haraldh]

I would suggest using https://github.com/zonyitoo/context-rs

[haraldh]

a history of all my tries could be found on https://github.com/haraldh/frenetic