[Userspace LL] zephyr: cpu: turn cpu_get_id() into a system call

[kv2019i]

cpu_get_id() ultimately reads a privileged special register via arch_proc_id() (e.g. the Xtensa PRID register). When low-atency pipelines run in user-space threads (CONFIG_SOF_USERSPACE_LL), this read is issued from user mode and faults. While many direct usages of cpu_get_id() have been removed from SOF codebase, multiple usages remain. As the remaining usages are mostly on less frequently used code paths (most via cpu_is_me() call, which is used in IPC handling), opt to keep the remaining cpu_get_id() calls and make the function available as a system call.

The system call machinery is gated on CONFIG_SOF_FULL_ZEPHYR_APPLICATION so unit-test builds keep the plain inline definition.

[kv2019i]

For context, this is part of #10558

[Copilot]

Pull request overview

This PR makes cpu_get_id() available as a Zephyr system call so user-mode threads (notably CONFIG_SOF_USERSPACE_LL low-latency pipelines) can safely read the current core ID without faulting on privileged register access, while keeping unit-test builds on the existing plain inline implementation via CONFIG_SOF_FULL_ZEPHYR_APPLICATION.

Changes:

• Adds a userspace verification/marshalling entrypoint for cpu_get_id() (z_vrfy_cpu_get_id() + mrsh include).
• Updates the Zephyr-side sof/lib/cpu.h to expose cpu_get_id() as __syscall and provide z_impl_cpu_get_id() inline implementations.
• Hooks the syscall header generation and syscall source into the Zephyr build.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
zephyr/syscall/cpu.c	Adds the syscall verification wrapper for cpu_get_id() and includes the generated marshalling code.
zephyr/include/sof/lib/cpu.h	Converts cpu_get_id() into a Zephyr syscall (when CONFIG_SOF_FULL_ZEPHYR_APPLICATION is enabled) and supplies z_impl_cpu_get_id() implementations.
zephyr/CMakeLists.txt	Registers the syscall header for generation and compiles the syscall handler when userspace is enabled.

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