LLVM warnings emitted for Xtensa target-cpu

[Noratrieb]

I ran rustc --target $TARGET for

• xtensa-esp32-espidf
• xtensa-esp32-none-elf
• xtensa-esp32s2-espidf
• xtensa-esp32s2-none-elf
• xtensa-esp32s3-espidf
• xtensa-esp32s3-none-elf

And for all of them, I got a warning that 'esp32' is not a recognized processor for this target (ignoring processor) (or esp32-s2 or esp32-s3 respectively).

Additionally for xtensa-esp32s2-none-elf, I also got '+forced-atomics' is not a recognized feature for this target (ignoring feature)

@MabezDev @SergioGasquez as Xtensa maintainers

[MabezDev]

Hi @Noratrieb, this intended, at least for a while.

LLVM patches are still being upstreamed and once they land, they take a while to propagate to the LLVM that rustc uses. We've applied these patches already to ease the upstreaming effort later, but also to reduce the patch set I have to maintain on our out of tree fork: https://github.com/esp-rs/rust which uses the full LLVM backend.

We can keep this open to track this, feel free to assign myself and @SergioGasquez