Use clang++ for C++ for the linux-x86_64-clang target

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

Configurations/10-main.conf

@@ -761,6 +761,7 @@ sub vms_info {
     "linux-x86_64-clang" => {
         inherit_from     => [ "linux-x86_64" ],
         cc               => "clang",
+        cxx              => "clang++",
         cflags           => add("-Wextra -Qunused-arguments"),
     },
     "linux-x32" => {