Add process identity primitives to std::sys

[LunaStev]

Tracked by #375 and represented in #378. Coordinate Linux architecture work with #370.

Scope

Add target-specific process identity primitives and a portable std::sys::process facade for:

• getuid and geteuid.
• getgid and getegid.
• setuid and setgid where the platform contract is stable.
• supplementary group queries when the required buffer contract is defined.

Linux and macOS implementations must use source-auditable constants and ABI signatures. Future Windows and BSD providers may expose the closest platform contract without pretending that Unix identities are universal.

Completion criteria

• Linux x86-64 wrappers have runtime smoke tests.
• RV64 constants and register contracts are integrated with Port the Linux standard-library syscall layer to RV64 #370.
• macOS behavior is tested on macOS CI.
• Unsupported operations have an explicit target-gated contract.
• The common facade does not leak architecture-specific syscall numbers.