Rollup merge of #131323 - jfrimmel:avr-inline-asm-clobber-abi, r=Amanieu

Support `clobber_abi` in AVR inline assembly

This PR implements the `clobber_abi` part necessary to eventually stabilize the inline assembly for AVR. This is tracked in #93335.
This is heavily inspired by the sibling-PR #131310 for the MSP430. I've explained my reasoning in the first commit message in detail, which is reproduced below for easier reviewing:

This follows the [ABI documentation] of AVR-GCC:

> The [...] call-clobbered general purpose registers (GPRs) are registers that might be destroyed (clobbered) by a function call.
>
> - **R18–R27, R30, R31**
>
>   These GPRs are call clobbered. An ordinary function may use them without restoring the contents. [...]
>
> - **R0, T-Flag**
>
>   The temporary register and the T-flag in SREG are also call-clobbered, but this knowledge is not exposed explicitly to the compiler (R0 is a fixed register).

Therefore this commit lists the aforementioned registers `r18–r27`, `r30` and `r31` as clobbered registers. Since the `r0` register (listed above as well) is not available in inline assembly at all (potentially because the AVR-GCC considers it a fixed register causing the register to never be used in register allocation and LLVM adopting this), there is no need to list it in the clobber list (the `r0`-variant is not even available). A comment was added to ensure, that the `r0` gets added to the clobber-list once the register gets usable in inline ASM.
Since the SREG is normally considered clobbered anyways (unless the user supplies the `preserve_flags`-option), there is no need to explicitly list a bit in this register (which is not possible to list anyways).

Note, that this commit completely ignores the case of interrupts (that are described in the ABI-specification), since every register touched in an ISR need to be saved anyways.

[ABI documentation]: https://gcc.gnu.org/wiki/avr-gcc#Call-Used_Registers

r? ``@Amanieu``

``@rustbot`` label +O-AVR

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