Support AVRTiny devices in AVR inline assembly

[taiki-e]

Note: This is still draft and this depends on #146900 which adds necessary target feature. (The first commit is from it.)

---

Follow-up to #131323.

AVR has devices that reduce the number of registers, similar to RISC-V's RV32E, which have different ABI than default. This PR supports such devices in inline assembly.

Refs: AVR-GCC docs https://gcc.gnu.org/wiki/avr-gcc#Reduced_Tiny

On the Reduced Tiny cores (16 GPRs only) several modifications to the ABI above apply:

Call-saved registers are: R18–R19, R28–R29.
Fixed Registers are R16 (tmp_reg) and R17 (zero_reg).

Registers used to pass arguments to functions and return values from functions are R25...R20 (instead of R25...R8).
Values that occupy more than 4 registers are returned in memory (instead of more than 8 registers).
There is only limited library support both from libgcc and AVR-LibC, for example there is no float support and no printf support.

r? @Amanieu

@rustbot label +O-AVR +A-inline-assembly

[tgross35]

Cc target maintainer @Patryk27

[Patryk27]

nit: would be nice to have a different message for register pairs, otherwise it takes a moment to parse what the compiler is trying to say

[Amanieu]

r=me once the target features land

[bors]

☔ The latest upstream changes (presumably #147645) made this pull request unmergeable. Please resolve the merge conflicts.