Implement SafeProcessHandle APIs for Linux and other Unixes

[Copilot]

• Update src/native/libs/configure.cmake with new feature detection checks for Linux (clone3, pidfd_send_signal, close_range, pdeathsig, sys_tgkill)
• Update src/native/libs/Common/pal_config.h.in with new #cmakedefine01 entries for the new features
• Update src/native/libs/System.Native/pal_process.c:
  • Add necessary includes for sys/syscall.h, linux/sched.h, sys/prctl.h
  • Define HAVE_PIDFD when HAVE_CLONE3 is available
  • Replace the #else (ENOTSUP) branch in SystemNative_SpawnProcess with fork/exec path using clone3/fork/vfork
  • Update SystemNative_SendSignal to use pidfd_send_signal when available
  • Add pidfd-based map_wait_status_pidfd using siginfo_t for pidfd path
  • Update SystemNative_TryGetExitCode to use waitid(P_PIDFD) when pidfd available
  • Update SystemNative_WaitForExitAndReap to use waitid(P_PIDFD) when pidfd available
  • Update SystemNative_TryWaitForExitCancellable to use poll with pidfd when available
  • Update SystemNative_TryWaitForExit to use poll with pidfd when available
  • Update SystemNative_OpenProcess to use waitid verification and pidfd_open when available
• Update test files:
  • Change SafeProcessHandleTests.Unix.cs from [PlatformSpecific(TestPlatforms.OSX)] to [PlatformSpecific(TestPlatforms.AnyUnix)]
  • Change SafeProcessHandleTests.cs from [PlatformSpecific(TestPlatforms.OSX | TestPlatforms.Windows)] to [PlatformSpecific(TestPlatforms.AnyUnix | TestPlatforms.Windows)]
• Native code compiles successfully
• All 395 tests pass (0 failures), including all 23 SafeProcessHandle tests
• Address review feedback: consolidate sys/syscall.h includes

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[adamsitnik]

@copilot So far I've found only one nit.