libc: add minimal C types

artiq/firmware/libc/Cargo.toml

@@ -0,0 +1,5 @@
+[package]
+name = "libc"
+version = "0.1.0"
+authors = ["M-Labs"]
+edition = "2018"

artiq/firmware/libc/src/lib.rs

@@ -0,0 +1,8 @@
+// Helper crate for dealing with c ffi
+#![allow(non_camel_case_types)]
+#![no_std]
+pub type c_char = i8;
+pub type c_int = i32;
+pub type size_t = usize;
+pub type uintptr_t = usize;
+pub type c_void = core::ffi::c_void;