Start test case for rjmp regression test

This commit introduces a minimal `![no_core]`-test case running on AVR,
that contains the MCWE mentioned in [129301]. The test case itself does
not have any assertions yet, but it shows the minimal set an language
items necessary within the test case.

[129301]: #129301 (comment)

tests/assembly/avr-rjmp-offsets.rs

@@ -0,0 +1,62 @@
+//@ compile-flags: -Copt-level=s --target=avr-unknown-gnu-atmega328 -C panic=abort
+//@ needs-llvm-components: avr
+//@ assembly-output: emit-asm
+
+#![feature(
+    no_core,
+    lang_items,
+    intrinsics,
+    rustc_attrs,
+    arbitrary_self_types,
+    asm_experimental_arch
+)]
+#![crate_type = "rlib"]
+#![no_core]
+
+#[rustc_builtin_macro]
+macro_rules! asm {
+    () => {};
+}
+
+use minicore::ptr;
+
+// CHECK-LABEL: pin_toggling
+#[no_mangle]
+pub fn pin_toggling() {
+    let port_b = 0x25 as *mut u8; // the I/O-address of PORTB
+    loop {
+        unsafe { ptr::write_volatile(port_b, 1) };
+        delay(500_0000);
+        unsafe { ptr::write_volatile(port_b, 2) };
+        delay(500_0000);
+    }
+}
+
+#[inline(never)]
+fn delay(_: u32) {
+    unsafe { asm!("nop") };
+}
+
+// FIXME: replace with proper minicore once available (#130693)
+mod minicore {
+    #[lang = "sized"]
+    pub trait Sized {}
+
+    #[lang = "copy"]
+    pub trait Copy {}
+    impl Copy for u32 {}
+    impl Copy for &u32 {}
+    impl<T: ?Sized> Copy for *mut T {}
+
+    pub mod ptr {
+        #[inline]
+        #[rustc_diagnostic_item = "ptr_write_volatile"]
+        pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
+            extern "rust-intrinsic" {
+                #[rustc_nounwind]
+                pub fn volatile_store<T>(dst: *mut T, val: T);
+            }
+            unsafe { volatile_store(dst, src) };
+        }
+    }
+}