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]: rust-lang#129301 (comment)

The issue was, that the disassembled label was placed one instruction
further down than expected. Therefore the test annotations check, that
the label is placed above the loop-contents (writing the one value, then
writing the other one).

Since the `tests/assembly` use `emit=asm`, the issue is not observable
as reported in the linked issue. Therefore the existing test case is
converted and a simple `rmake`-test is added. The test only checks, if
the correct `rjmp`-offset is used.

The new `rmake`-content asserts the exact assembly sequence for the loop
preventing false-negatives if some instructions would change and thus
the label offset might need to change.

This fixes the [build error] caused by the `avr-gcc` (used as linker)
not being available in the Rust CI. This is a viable solution, which
shows the wrong/right behavior and, since no functions from `libgcc` are
called, does not produce errors. This was discussed [here]. Another
small problem is, that `lld` doesn't link the correct startup-code by
default. This is not a problem for this test (since it does not actually
use anything the startup code is needed for (no variables, no stack, no
interrupts)), but this causes the `main`-function to be removed by the
default flag `--gc-sections`. Therefore the `rmake`-driver also adds the
linker flag `--entry=main` to mark the `main`-function as the entry
point and thus preventing it from getting removed. The code would work
on a real AVR device.

[build error]: rust-lang#131755 (comment)
[here]: rust-lang#131755 (comment)