Build std for stage0
There was a race between the following commits:
Which introduced a bug that causes Kokoro to fail.
@@ -3,3 +3,8 @@ target = "x86_64-unknown-none"
[target.x86_64-unknown-none]
rustflags = "-C relocation-model=static -C code-model=large"
+
+[unstable]
+# We need to build std / core because of the code model. This may be removed if / when it's possible to build stage0 without it in the future.
+build-std = ["core"]
+build-std-features = ["compiler-builtins-mem"]
