implement Stderr in vexos PAL

Author: Tropix126

library/std/src/sys/pal/vexos/stdio.rs

@@ -72,10 +72,26 @@ impl Stderr {
 
 impl io::Write for Stderr {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        Ok(buf.len())
+        let written =
+            unsafe { vex_sdk::vexSerialWriteBuffer(STDIO_CHANNEL, buf.as_ptr(), buf.len() as u32) };
+
+        if written < 0 {
+            return Err(io::Error::new(
+                io::ErrorKind::Uncategorized,
+                "Internal write error occurred.",
+            ));
+        }
+
+        Ok(written as usize)
     }
 
     fn flush(&mut self) -> io::Result<()> {
+        unsafe {
+            while (vex_sdk::vexSerialWriteFree(STDIO_CHANNEL) as usize) != STDOUT_BUF_SIZE {
+                vex_sdk::vexTasksRun();
+            }
+        }
+
         Ok(())
     }
 }

src/doc/rustc/src/platform-support/armv7a-vex-v5.md

@@ -30,7 +30,7 @@ This target generates binaries in the ELF format that may uploaded to the brain
 This includes:
 
 - `std::time`, not including `SystemTime` as the SDK does not provide absolute time information.
-- `std::io`, including `stdin()` and `stdout()`, but not `stderr()`, as stderr does not exist on this platform.
+- `std::io`, including `stdin`/`stdout`/`stderr`. `stdout` and `stderr` are both written to USB channel 1 on this platform and are not differentiated.
 - `std::fs`, with the exception of directory reading and file deletion, due to public SDK limitations.
 - modules which do not need to interact with the OS beyond allocation,
   such as `std::collections`, `std::hash`, `std::future`, `std::sync`, etc.