[llvm][Support] Use __NR_gettid on Linux for compat with older glibc (#120007)

Author: brad0

llvm/lib/Support/Unix/Threading.inc

@@ -141,7 +141,7 @@ uint64_t llvm::get_threadid() {
 #elif defined(__ANDROID__)
   return uint64_t(gettid());
 #elif defined(__linux__)
-  return uint64_t(syscall(SYS_gettid));
+  return uint64_t(syscall(__NR_gettid));
 #else
   return uint64_t(pthread_self());
 #endif