relibc: fix round-8 deferred stubs in linux platform

signal.rs: sigqueue now fills si_pid/si_uid with real getpid()/getuid()
  instead of zeros, allowing receivers to identify the sender.

mod.rs: exit_thread now frees the thread stack via munmap before calling
  __NR_exit (which exits just the calling thread on Linux, not the
  process). Previously it called process::exit(0) which leaked the stack
  and had misleading semantics.

mod.rs: rlct_clone for aarch64 is now implemented with proper inline
  assembly for the clone syscall (SYS_CLONE=220). The child pops the
  function pointer and arguments from the prepared stack, calls
  new_thread_shim, then exits via __NR_exit (93). Previously this was
  todo!() which panicked on thread creation.

src/platform/linux/mod.rs

@@ -1,4 +1,4 @@
-#[cfg(target_arch = "x86_64")]
+#[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
 use core::arch::asm;
 
 use super::{Pal, types::*};
@@ -165,9 +165,15 @@ impl Pal for Sys {
         }
         unreachable!();
     }
-    unsafe fn exit_thread(_stack_base: *mut (), _stack_size: usize) -> ! {
+    unsafe fn exit_thread(stack_base: *mut (), stack_size: usize) -> ! {
-        // TODO
+        // Free the thread's stack before exiting. On Linux, __NR_exit only
-        Self::exit(0)
+        // terminates the calling thread (not the process); __NR_exit_group
+        // terminates all threads. We want thread-only exit here.
+        if !stack_base.is_null() && stack_size > 0 {
+            let _ = e_raw(syscall!(MUNMAP, stack_base, stack_size));
+        }
+        unsafe { syscall!(EXIT, 0); }
+        unreachable!();
     }
 
     fn fchdir(fildes: c_int) -> Result<()> {
@@ -627,7 +633,78 @@ impl Pal for Sys {
         stack: *mut usize,
         _os_specific: &mut OsSpecific,
     ) -> Result<crate::pthread::OsTid> {
-        todo!("rlct_clone not implemented for aarch64 yet")
+        const SYS_CLONE_AARCH64: usize = 220;
+        const SYS_EXIT_AARCH64: usize = 93;
+
+        let flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD;
+        let pid;
+        unsafe {
+            asm!(
+                "svc #0",
+
+                // If x0 != 0 we are the parent; jump to return.
+                "cbnz x0, 2f",
+
+                // Child: pop function pointer and args from the stack
+                // that the pthread code prepared.
+                "ldr x9, [sp], #8",  // x9 = new_thread_shim
+                "ldr x0, [sp], #8",  // x0 = start_routine
+                "ldr x1, [sp], #8",  // x1 = arg
+                "ldr x2, [sp], #8",  // x2 = tcb
+                "ldr x3, [sp], #8",  // x3 = sync_mutex
+                "ldr x4, [sp], #8",  // skip unused
+                "ldr x5, [sp], #8",  // skip unused
+                "ldr x6, [sp], #8",  // skip aarch64 alignment pad
+
+                "blr x9",            // call new_thread_shim(...)
+
+                // new_thread_shim returns !, but guard just in case.
+                "mov x8, {exit_nr}",
+                "mov x0, xzr",
+                "svc #0",
+                "udf #0",
+
+                "2:",
+
+                exit_nr = const SYS_EXIT_AARCH64,
+
+                inout("x0") flags => pid,
+                inout("x1") stack => _,
+                in("x2") 0usize,
+                in("x3") 0usize,
+                in("x4") 0usize,
+                in("x8") SYS_CLONE_AARCH64,
+
+                out("x5") _,
+                out("x6") _,
+                out("x7") _,
+                out("x9") _,
+                out("x10") _,
+                out("x11") _,
+                out("x12") _,
+                out("x13") _,
+                out("x14") _,
+                out("x15") _,
+                out("x16") _,
+                out("x17") _,
+                out("x18") _,
+                out("x19") _,
+                out("x20") _,
+                out("x21") _,
+                out("x22") _,
+                out("x23") _,
+                out("x24") _,
+                out("x25") _,
+                out("x26") _,
+                out("x27") _,
+                out("x28") _,
+
+                options(nostack),
+            );
+        }
+        let tid = e_raw(pid)?;
+
+        Ok(crate::pthread::OsTid { thread_id: tid })
     }
 
     unsafe fn rlct_kill(os_tid: crate::pthread::OsTid, signal: usize) -> Result<()> {

src/platform/linux/signal.rs

@@ -5,7 +5,7 @@ use core::{
 
 use super::{
     super::{
-        PalSignal,
+        Pal, PalSignal,
         types::{c_int, pid_t},
     },
     Sys, e_raw,
@@ -39,10 +39,10 @@ impl PalSignal for Sys {
             si_addr: core::ptr::null_mut(),
             si_code: SI_QUEUE,
             si_errno: 0,
-            si_pid: 0, // TODO: GETPID?
+            si_pid: Self::getpid(),
             si_signo: sig,
             si_status: 0,
-            si_uid: 0, // TODO: GETUID?
+            si_uid: Self::getuid(),
             si_value: val,
         };
         e_raw(unsafe { syscall!(RT_SIGQUEUEINFO, pid, sig, addr_of!(info)) }).map(|_| ())