embassy-sync: add ThreadModeRawMutex support for riscv32

[cong-or]

Fix ThreadModeRawMutex behavior on RISC-V (#5133)

ThreadModeRawMutex only works on cortex_m right now, so it can’t be used on riscv32 targets.
The RISC-V executor is thread-only for now, so in_thread_mode() just returns true.

If we add an interrupt executor for RISC-V in the future, this will need proper tracking—but that’s a separate PR.

Testing

• Builds cleanly on riscv32imac-unknown-none-elf
• All host tests pass

[bugadani]

That's false, esp-hal/esp-rtos provides an interrupt executor for risc-v chips.

[cong-or]

@bugadani Would an AtomicBool that the interrupt executor sets on entry/exit work? Or does esp-hal already have a mechanism for tracking interrupt context that this should hook into?

[bugadani]

There is no standard interrupt controller on RISC-V chips, and embassy-sync shouldn't rely on anything ESP32-specific. I'm not aware of an implementation-independent way to check if code is running in a thread context

[cong-or]

Wouldn't an AtomicBool set/cleared by the executor itself be implementation-independent? It doesn't need to know anything about the interrupt controller — just that the executor entered/exited interrupt context.