Import the CloudABI system call bindings into the libstd tree.

These automatically generated Rust source files allow us to invoke
system calls within CloudABI processes. These will be used by libstd to
implement primitives for I/O, threading, etc.

These source files are normally part of the 'cloudabi' crate. In the
case of libstd, we'd better copy them into the source tree, as having
external dependencies in libstd is a bit messy. Original source files
can be found here:

    https://github.com/NuxiNL/cloudabi/tree/master/rust

src/libstd/sys/cloudabi/abi/bitflags.rs

@@ -0,0 +1,51 @@
+// Copyright (c) 2018 Nuxi (https://nuxi.nl/) and contributors.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions
+// are met:
+// 1. Redistributions of source code must retain the above copyright
+//    notice, this list of conditions and the following disclaimer.
+// 2. Redistributions in binary form must reproduce the above copyright
+//    notice, this list of conditions and the following disclaimer in the
+//    documentation and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+// SUCH DAMAGE.
+
+// Appease Rust's tidy.
+// ignore-license
+
+#[cfg(feature = "bitflags")]
+#[macro_use]
+extern crate bitflags;
+
+// Minimal implementation of bitflags! in case we can't depend on the bitflags
+// crate. Only implements `bits()` and a `from_bits_truncate()` that doesn't
+// actually truncate.
+#[cfg(not(feature = "bitflags"))]
+macro_rules! bitflags {
+  (
+    $(#[$attr:meta])*
+    pub struct $name:ident: $type:ty {
+      $($(#[$const_attr:meta])* const $const:ident = $val:expr;)*
+    }
+  ) => {
+    $(#[$attr])*
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    pub struct $name { bits: $type }
+    impl $name {
+      $($(#[$const_attr])* pub const $const: $name = $name{ bits: $val };)*
+      pub fn bits(&self) -> $type { self.bits }
+      pub fn from_bits_truncate(bits: $type) -> Self { $name{ bits } }
+    }
+  }
+}