Add support for xtensa-esp32 target

[sansmoraxz]

Fixes: mlua-rs/mlua#642

[sansmoraxz]

cc: @khvzak

[khvzak]

@sansmoraxz how hard would be adding CI support to test this change? I want to make sure that next Lua versions (e.g. 5.5) or changes in code don't break anything.

[sansmoraxz]

Not entirely sure tbh. I have tested physically against an ESP32 board with esp-idf toolchain.

There's xtensa compiler toolchain available as github actions. https://github.com/esp-rs/xtensa-toolchain

Not sure if it's possible to execute test cases via some sort of QEMU emulation.

In official repos I see them using xcheck while in some normal cargo commands. Maybe it has something to do with the presence of std environments.

See:

• https://github.com/esp-rs/esp-idf-sys/blob/master/.github/workflows/ci.yml
• https://github.com/esp-rs/esp-hal/blob/main/.github/workflows/ci.yml

But those are mostly linting and build success / failures. Not test case execution in those specific architecture (only host)

Perhaps @ivmarkov or @Vollbrecht could pitch in.