Add Xtensa asm!() esp32 target

[KerryRJ]

Located registers in the IDF source, so this probably needs checking by someone knowledgeable in this area as I am likely missing something. I have only tested using the "xtensa-rust-quickstart" after applying a minor fix to "esp32-hal".

cargo espflash --chip esp32 --example alloc --speed 115200 --tool=cargo --features="xtensa-lx-rt/lx6,xtensa-lx/lx6,esp32-hal" COM3

Edit: Also modified the config.toml file.

[MabezDev]

Thanks! I've also been working on this locally, which I will publish soon - it has support for excluding registers on targets that don't support them (i.e no fpu on esp8266).

[KerryRJ]

I was just thinking about the register exclusions for other targets earlier today but was unsure how to approach it. Just implemented what I was fiddling with over the last month as global_asm!() is failing with Rust 1.54. I was just busy migrating llvm_asm!() in xtensa-lx-rt.

[MabezDev]

I've just pushed my changes in #68.

I was just busy migrating llvm_asm!() in xtensa-lx-rt

How are you getting on? That's next on my list - perhaps we can work together on it.

[KerryRJ]

I completed the changes (they'll need checking as I wasn't too sure about what to do with 2/3 of them) and was looking at migrating esp32 to use build.rs, instead of the makefile (not very cross platform), so that I could run the changes in xtensa-lx-rt through the esp32-hal examples. Do you have an idea on improving cross-platform compilation of the esp32 crate?

[KerryRJ]

Closing in favour of #68