Auto merge of #31078 - nbaksalyar:illumos, r=alexcrichton

This pull request adds support for [Illumos](http://illumos.org/)-based operating systems: SmartOS, OpenIndiana, and others. For now it's x86-64 only, as I'm not sure if 32-bit installations are widespread. This PR is based on #28589 by @potatosalad, and also closes #21000, #25845, and #25846.

Required changes in libc are already merged: https://github.com/rust-lang-nursery/libc/pull/138

Here's a snapshot required to build a stage0 compiler:
https://s3-eu-west-1.amazonaws.com/nbaksalyar/rustc-sunos-snapshot.tar.gz
It passes all checks from `make check`.

There are some changes I'm not quite sure about, e.g. macro usage in `src/libstd/num/f64.rs` and `DirEntry` structure in `src/libstd/sys/unix/fs.rs`, so any comments on how to rewrite it better would be greatly appreciated.

Also, LLVM configure script might need to be patched to build it successfully, or a pre-built libLLVM should be used. Some details can be found here: https://llvm.org/bugs/show_bug.cgi?id=25409

Thanks!

r? @brson

src/libsyntax/abi.rs

@@ -28,6 +28,7 @@ pub enum Os {
     OsNetbsd,
     OsOpenbsd,
     OsNaCl,
+    OsSolaris,
 }
 
 #[derive(PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Clone, Copy, Debug)]
@@ -147,6 +148,7 @@ impl fmt::Display for Os {
             OsNetbsd => "netbsd".fmt(f),
             OsOpenbsd => "openbsd".fmt(f),
             OsNaCl => "nacl".fmt(f),
+            OsSolaris => "solaris".fmt(f),
         }
     }
 }