fastbuild: replace -gmlt with -O0

- -gmlt is a noop in zig, because -g is a default anyway.
- -O0 was fixed in ziglang/zig 02ba3cb25cc8ee2637c6fb7bcb935722490ba05c

toolchain/zig_toolchain.bzl

@@ -75,7 +75,7 @@ def _compilation_mode_features(ctx):
                 actions = actions,
                 flag_groups = [
                     flag_group(
-                        flags = ["-gmlt", "-fno-lto", "-Wl,-S"],
+                        flags = ["-fno-lto", "-Wl,-S","-O0"],
                     ),
                 ],
             ),