[Threading] Use raw gettid syscall for older glibc compatibility

`gettid` wrapper was added in glibc 2.30, and Ubuntu 18.04 uses glibc
2.27, so use raw syscall instead of it.

include/swift/Threading/Impl/Linux/ulock.h

@@ -46,7 +46,7 @@ typedef std::int32_t ulock_t;
 inline int ulock_get_tid(void) {
   static __thread int tid;
   if (tid == 0)
-    tid = gettid();
+    tid = syscall(SYS_gettid);
   return tid;
 }