Add atomic enabled targets in the Rust compiler?

[MabezDev]

Using json target definitions is kinda awkward.

What would be the naming convention of such a target?

[MabezDev]

Perhaps instead we should default to using atomic cas in the s2 (perhaps a reference to the trap handler).

Then we can either:

• add a target called xtensa-esp32s2noatomics-none-elf to the compiler
• write docs to create a json file for the non-atomic use-case

[MabezDev]

Instead I found a better way with the following rustflags.

rustflags = [
# enable the atomic codegen option for Xtensa
"-C", "target-feature=+s32c1i",

# tell the core library have atomics even though it's not specified in the target definition
"--cfg", 'target_has_atomic="8"',
"--cfg", 'target_has_atomic="16"',
"--cfg", 'target_has_atomic="32"',
"--cfg", 'target_has_atomic="ptr"',
]

We should probably do the same for the RISCV atomic emulation trap handler, just to avoid confusion about what the current c3 target is.