[Bug c++/84497] link errors with trivial external thread_local variables

[P at draigBrady dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84497

--- Comment #1 from Pádraig Brady <P at draigBrady dot com> ---
Created attachment 43479
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43479&action=edit
Proposed patch and testcase. This passes the full testsuite