fasta-redux and threadring failing on bots

[brson]

http://buildbot.rust-lang.org/builders/incoming-full-mac/builds/1002/steps/test/logs/stdio

Threadring is running out of memory and fasta-redux does not typecheck on x86. I will xfail them.

[brson]

threadring OOM's because of a subtle bug in fast_ffi that proceeds like so:

1. task calls C using fast_ffi, borrowing a big stack from the scheduler.
2. task returns from C and places the big stack on the task-local stack segment list
3. task calls further Rust functions that require growing the stack, and for this reuses the big stack
4. task yields, failing to return the big stack to the scheduler.
5. repeat 500+ times and OOM

[catamorphism]

@brson Do you think this needs to be on 0.7?

[brson]

Fixed.