Merge LLVM DragonFly segmented stack support

[mneumann]

This means that rustc will finally be able to target DragonFly out-of-the-box, i.e. I can use a nightly snapshot to cross-compile for DragonFly without the need to patch it myself.

This commit: http://llvm.org/viewvc/llvm-project?view=revision&revision=224936
On github: llvm-mirror/llvm@a21d820

http://reviews.llvm.org/D4705

[mneumann]

I can issue a pull request towards rust-lang/llvm if someone tells me towards which branch (master?) I should apply the patches.

[mneumann]

Fixed in #20472.