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: rust-lang/libc#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

Author: bors

lib.rs

@@ -927,7 +927,8 @@ fn get_concurrency() -> usize {
     #[cfg(any(target_os = "linux",
               target_os = "macos",
               target_os = "ios",
-              target_os = "android"))]
+              target_os = "android",
+              target_os = "solaris"))]
     fn num_cpus() -> usize {
         unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) as usize }
     }