Auto merge of #47089 - EdSchouten:cloudabi-is-not-unix, r=kennytm

Don't announce CloudABI as being UNIX.

This was originally brought in, because the definitions are based on
those of FreeBSD, Linux, etc. Even though CloudABI is based on POSIX, it
uses a subset that is so small that it's not reasonable to call it POSIX.

Now that I'm porting libstd, I'm running into some spots where I have to
explicitly disable code paths that were enabled by cfg(unix).

src/librustc_back/target/cloudabi_base.rs

@@ -23,7 +23,7 @@ pub fn opts() -> TargetOptions {
 
     TargetOptions {
         executables: true,
-        target_family: Some("unix".to_string()),
+        target_family: None,
         linker_is_gnu: true,
         pre_link_args: args,
         position_independent_executables: true,