Add basic ARM rust freestanding binary

Following the modern version of Philipp Oppermann's blog-tutorial. Build
with `cargo build --target thumbv7em-none-eabihf`

os-in-rust/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "os-in-rust"
+version = "0.1.0"

os-in-rust/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "os-in-rust"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[profile.dev]
+panic = "abort"
+
+[profile.release]
+panic = "abort"
+
+[dependencies]

os-in-rust/src/main.rs

@@ -0,0 +1,15 @@
+#![no_std]
+#![no_main]
+
+use core::panic::PanicInfo;
+
+#[panic_handler]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[no_mangle]
+pub extern "C" fn _start() -> ! {
+    loop {}
+}
+