Install: stop pulling every submodule when a required one is missing

check_and_update_submodules() ran a bare `git submodule update --init
--recursive` whenever a required submodule was absent, which fetched all
~24 submodules including llama.cpp and mlx, even though the default build
uses neither. Update only the missing required paths instead.

flatcc is built unconditionally by third-party/CMakeLists.txt but was only
ever fetched as a side effect of the bulk update, so add it to
REQUIRED_SUBMODULES. Mark llama.cpp shallow like mlx to trim its clone.

This PR was authored with the assistance of Claude Code.

Author: JakeStevens

.gitmodules

@@ -71,3 +71,4 @@
 [submodule "backends/native/third-party/llama.cpp"]
 	path = backends/native/third-party/llama.cpp
 	url = https://github.com/ggml-org/llama.cpp.git
+	shallow = true

install_executorch.py

@@ -96,6 +96,7 @@ def clean():
     "cpuinfo": "CMakeLists.txt",
     "eigen": "CMakeLists.txt",
     "flatbuffers": "CMakeLists.txt",
+    "flatcc": "CMakeLists.txt",
     "FP16": "CMakeLists.txt",
     "FXdiv": "CMakeLists.txt",
     "gflags": "CMakeLists.txt",
@@ -145,7 +146,8 @@ def check_folder(folder: str, file: str) -> bool:
         try:
             subprocess.check_call(["git", "submodule", "sync", "--recursive"])
             subprocess.check_call(
-                ["git", "submodule", "update", "--init", "--recursive"]
+                ["git", "submodule", "update", "--init", "--recursive", "--"]
+                + list(missing_submodules)
             )
         except subprocess.CalledProcessError as e:
             logger.error(f"Error updating submodules: {e}")