Rollup merge of #75749 - ehuss:consolidate-sys, r=alexcrichton

Consolidate some duplicate code in the sys modules.

This consolidates some modules which were duplicated throughout the sys module. The intent is to make it easier to update and maintain this code. This mainly affects the wasi, sgx, and "unsupported" targets.

I explicitly skipped hermit, cloudabi, and vxworks. These tier-3 targets have copied large sections of the sys tree. I don't think they should have, but I don't want to put effort into changing them. It also doesn't help that there aren't any scripts or instructions for building them.

There are still sections of duplicate code here and there, but this PR covers the easy parts where entire modules are the same.

Author: Dylan-DPC

library/std/src/sys/sgx/fs.rs

@@ -17,14 +17,18 @@ pub mod condvar;
 pub mod env;
 pub mod ext;
 pub mod fd;
+#[path = "../unsupported/fs.rs"]
 pub mod fs;
+#[path = "../unsupported/io.rs"]
 pub mod io;
 pub mod memchr;
 pub mod mutex;
 pub mod net;
 pub mod os;
 pub mod path;
+#[path = "../unsupported/pipe.rs"]
 pub mod pipe;
+#[path = "../unsupported/process.rs"]
 pub mod process;
 pub mod rwlock;
 pub mod stack_overflow;