rustc: Implement stack probes for x86

This commit implements stack probes on x86/x86_64 using the freshly landed
support upstream in LLVM. The purpose of stack probes here are to guarantee a
segfault on stack overflow rather than having a chance of running over the guard
page already present on all threads by accident.

At this time there's no support for any other architecture because LLVM itself
does not have support for other architectures.

src/test/codegen/stack-probes.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-arm
+// ignore-wasm
+// ignore-emscripten
+// ignore-windows
+// no-system-llvm
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn foo() {
+// CHECK: @foo() unnamed_addr #0
+// CHECK: attributes #0 = { {{.*}}"probe-stack"="__rust_probestack"{{.*}} }
+}