[PATCH] Move the segmented stack switch to a function attribute

[Alex Crichton]

Awhile back, Luqman Aden made a request [1] to merge a
"no-split-stack" function attribute for disabling the split-stack
prologue in an otherwise segmented-stack-enabled program. I believe it
was later recommended to instead remove the command line switch and
use an opt-in system instead (with a "split-stack" attribute for
functions) [2].

I asked Luqman if it was ok for me to rebase his patch and try to
re-submit it, and he said that I had the green light. I've rebased his
work from [2] on today's LLVM, and addressed a few comments that were
raised on the review.

I've only recently found the phabricator instance for LLVM, so if it
would be more helpful to move the patch onto there, please just let me
know!

[1] - http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20131007/190913.html
[2] - http://llvm-reviews.chandlerc.com/D1905