std: xous: add os-specific ffi calls

Xous has no C FFI. Instead, all FFI is done via syscalls that are
specified in Rust. Add these FFI calls to libstd, as well as some of the
currently-supported syscalls.

This enables Rust programs to interact with the Xous operating system
while avoiding adding an extra dependency to libstd.

Signed-off-by: Sean Cross <sean@xobs.io>

Author: xobs

library/std/src/os/mod.rs

@@ -146,6 +146,8 @@ pub mod vita;
 pub mod vxworks;
 #[cfg(target_os = "watchos")]
 pub(crate) mod watchos;
+#[cfg(target_os = "xous")]
+pub mod xous;
 
 #[cfg(any(unix, target_os = "wasi", doc))]
 pub mod fd;