zig cc: handle the -r flag

This makes -r treated the same as -c which is to output an object file.
Zig's ELF linker code already handles multiple object files into an
object file with the -r flag to LLD.

closes #11683

tools/update_clang_options.zig

@@ -34,6 +34,10 @@ const known_options = [_]KnownOpt{
         .name = "c",
         .ident = "c",
     },
+    .{
+        .name = "r",
+        .ident = "r",
+    },
     .{
         .name = "l",
         .ident = "l",