feat(thread): futex + TLS + pthread_self substrate

src/thread/x86_64/__set_thread_area.s

@@ -0,0 +1,34 @@
+/*
+ * vlibc — x86_64 thread-pointer installer (todo 44).
+ *
+ * __set_thread_area(tp): install tp as the calling thread's FS base via
+ * arch_prctl(ARCH_SET_FS). After this, %fs-relative TLS offsets resolve
+ * against tp exactly as the linker baked them (see src/start/tcb.h for the
+ * placement contract), and __builtin_thread_pointer() / %fs:0 returns tp.
+ *
+ * Must be executed IN the thread whose FS base is being set: arch_prctl
+ * operates on the current task, so pthread_create (#45) calls this from the
+ * new thread's entry point, never from the parent.
+ *
+ * Return convention: the raw syscall result is left in %eax — 0 on success,
+ * a negative errno on failure. No errno write happens here; the caller maps
+ * the result however its context needs (this mirrors musl's
+ * src/thread/x86_64/__set_thread_area.s, which also returns raw).
+ *
+ * arch_prctl codes (kernel UAPI asm/prctl.h): ARCH_SET_FS 0x1002,
+ * SYS_arch_prctl 158. Kept as literals because there is no internal header
+ * for them yet; src/start/tls.c defines the same values locally.
+ */
+
+.text
+.global __set_thread_area
+.hidden __set_thread_area
+.type __set_thread_area,@function
+__set_thread_area:
+	mov %rdi,%rsi
+	movl $0x1002,%edi
+	movl $158,%eax
+	syscall
+	ret
+.size __set_thread_area,.-__set_thread_area
+.section .note.GNU-stack,"",@progbits