This test is 'dg-do compile', so require tls instead of tls_runtime.
This enables it on targets such as arm-none-eabi configured with
--enable-threads=no.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constinit16.C: Require tls.
// { dg-do compile { target c++20 } }
// { dg-add-options tls }
// { dg-require-alias "" }
-// { dg-require-effective-target tls_runtime }
+// { dg-require-effective-target tls }
// { dg-final { scan-assembler-not "_ZTH17mythreadlocalvar1" } }
// { dg-final { scan-assembler "_ZTH17mythreadlocalvar2" } }
// { dg-final { scan-assembler-not "_ZTH17mythreadlocalvar3" } }