ESP-IDF targets' Atomic*64 is not lock-free

[taiki-e]

max_atomic_width of these targets is currently set to 64

rust/compiler/rustc_target/src/spec/riscv32imac_esp_espidf.rs

Line 20 in 0237aa3

max_atomic_width: Some(64),

rust/compiler/rustc_target/src/spec/riscv32imc_esp_espidf.rs

Line 23 in 0237aa3

max_atomic_width: Some(64),

However, as mentioned in #115577 (comment), esp-idf's 64-bit atomic builtins use global spinlocks on multi-core systems:
https://github.com/espressif/esp-idf/blob/3b748a6cb76c2db7c6368a0dea32a88bc58bc44d/components/newlib/stdatomic.c#L64

And it violates what the standard library is intended to guarantee.

https://doc.rust-lang.org/nightly/std/sync/atomic/index.html#portability

All atomic types in this module are guaranteed to be lock-free if they’re available. This means they don’t internally acquire a global mutex.

Since riscv32 does not have 64-bit atomic instructions, I do not believe there is any way to fix this problem other than setting max_atomic_width of these targets to 32.

cc @ivmarkov @MabezDev (mentioned because you both are target maintainers)

---

@rustbot label +A-atomic +I-unsound

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-low