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/libstd/sys/unix/stack_overflow.rs

@@ -39,6 +39,7 @@ impl Drop for Handler {
           target_os = "bitrig",
           target_os = "dragonfly",
           target_os = "freebsd",
+          target_os = "solaris",
           all(target_os = "netbsd", not(target_vendor = "rumprun")),
           target_os = "openbsd"))]
 mod imp {
@@ -179,6 +180,7 @@ mod imp {
               target_os = "bitrig",
               target_os = "dragonfly",
               target_os = "freebsd",
+              target_os = "solaris",
               all(target_os = "netbsd", not(target_vendor = "rumprun")),
               target_os = "openbsd")))]
 mod imp {