[libc] Add missing threads dependency to AArch64 startup implementation

Summary:
The changes in D146184 made the integration tests use the inhereted
dependencies from the startup code like a normal target. For the AArch64
target this resulted in the threads depenency not being pulled in
because it was not present in the original code.

libc/startup/linux/aarch64/CMakeLists.txt

@@ -6,6 +6,7 @@ add_startup_object(
     libc.config.linux.app_h
     libc.include.sys_mman
     libc.include.sys_syscall
+    libc.src.__support.threads.thread
     libc.src.__support.OSUtil.osutil
     libc.src.stdlib.exit
     libc.src.stdlib.atexit