[lld] Canonicalize HAVE_LIBXAR

I think this should unbreak the build after D100650...

GitOrigin-RevId: 3e1045ec04a033c688aa40e961e26c38446f1139
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 6bf0435..8a7ca86 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -11,6 +11,7 @@
   LLVM_ENABLE_ZLIB
   LLVM_ENABLE_LIBXML2
   LLD_DEFAULT_LD_LLD_IS_MINGW
+  HAVE_LIBXAR
   )
 
 configure_lit_site_cfg(