build: replace wasm patch with third_party/qemu submodule

The 7.7k-line qemu-wasm patch was really a fork, so vendor it as a submodule
(pinned at the xteink QEMU fork) and build it directly. Removes the
clone/commit-check/patch/reset machinery, the redundant web-dev/patch targets,
and 0002-qemu-wasm.patch. Editing third_party/qemu now recompiles incrementally
via plain `make web`. Native build keeps 0001-xteink-machine.patch.

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "third_party/qemu"]
+	path = third_party/qemu
+	url = git@ssh.gitea.va.reichard.io:evan/qemu-xteink.git
+	branch = main