crt: Mark _tls_used as used for LTO

02b4df13b1aac0dba0a35e87f1a536594fd35edd added attribute used to
_CRTALLOC, applying it automatically on most such special variables
that need to be retained during LTO. _tls_used is also such a variable,
but it's not located in any special section; thus apply the attribute
directly.

Signed-off-by: Martin Storsjö <martin@martin.st>