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>

mingw-w64-crt/crt/tlssup.c

diff --git a/mingw-w64-crt/crt/tlssup.c b/mingw-w64-crt/crt/tlssup.c
index 47beb27..1b9d7b4 100644
--- a/mingw-w64-crt/crt/tlssup.c
+++ b/mingw-w64-crt/crt/tlssup.c
@@ -44,6 +44,7 @@
 _CRTALLOC(".CRT$XLA") PIMAGE_TLS_CALLBACK __xl_a = 0;
 _CRTALLOC(".CRT$XLZ") PIMAGE_TLS_CALLBACK __xl_z = 0;
 
+__attribute__((used))
 const IMAGE_TLS_DIRECTORY _tls_used = {
   (ULONG_PTR) &_tls_start, (ULONG_PTR) &_tls_end,
   (ULONG_PTR) &_tls_index, (ULONG_PTR) (&__xl_a+1),