Make libpanic_unwind build on CloudABI.

CloudABI uses LLVM's libunwind for stack unwinding. There was a small
bug that went by unnoticed, namely that it was not built with -fno-rtti.
This caused it to (indirectly) depend on the entire C++ runtime.

Now that that issue has been resolved, it is also perfectly fine to make
use of this library for programming languages other than C++.

src/libpanic_unwind/lib.rs

@@ -68,6 +68,7 @@
 
 // i686-pc-windows-gnu and all others
 #[cfg(any(all(unix, not(target_os = "emscripten")),
+          target_os = "cloudabi",
           target_os = "redox",
           all(windows, target_arch = "x86", target_env = "gnu")))]
 #[path = "gcc.rs"]