[fastbuild] use -fno-lto

This is very cache-unfriendly, because quite a lot of work is done
during the final linking stage. Let's see how this improves performance
of fastbuild.

toolchain/zig_toolchain.bzl

@@ -75,7 +75,7 @@ def _compilation_mode_features(ctx):
                 actions = actions,
                 flag_groups = [
                     flag_group(
-                        flags = ["-gmlt", "-Wl,-S"],
+                        flags = ["-gmlt", "-fno-lto", "-Wl,-S"],
                     ),
                 ],
             ),