Move llvm from deps to deps_os

DEPS

 use_relative_paths = True
 
 # Dependencies on outside packages.
 #
 deps = {
   "common": "https://skia.googlesource.com/common.git@c282fe0b6e392b14f88d647cbd86e1a3ef5498e0",
 
   # There is some duplication here that might be worth cleaning up:
   #   - can use use our existing t_p/e/libjpeg instead of pulling it for Android?
 
@@ skipping 38 matching lines @@
   "third_party/externals/microhttpd" : "https://android.googlesource.com/platform/external/libmicrohttpd@748945ec6f1c67b7efc934ab0808e1d32f2fb98d",
 
   # Mojo SDK for Mojo RPC experiments.
   "third_party/externals/mojo/public" : "https://chromium.googlesource.com/external/github.com/domokit/mojo_sdk@172be18ea745ab29aea132e6cb952e726dc32543",
   
   # shaderc for the vulkan backend
   "third_party/externals/shaderc2" : "https://github.com/google/shaderc.git@f76848a36d539888828006b9b7c088b3f362b607",
   "third_party/externals/shaderc2/third_party/googletest" : "https://github.com/google/googletest.git@ff07a5de0e81580547f1685e101194ed1a4fcd56",
   "third_party/externals/shaderc2/third_party/glslang" : "https://github.com/bsalomon/glslang.git@5dc8a76265ee31733b5a765e034ad7f5097a3cbe",
   "third_party/externals/shaderc2/third_party/spirv-tools" : "https://github.com/KhronosGroup/SPIRV-Tools.git@c31a31942b3697f2f71148555e5ffa298a572dbd",
+}
 
-  # xSAN bots build Clang from scratch.
-  "third_party/externals/llvm": "https://llvm.googlesource.com/llvm@release_38",
-  "third_party/externals/llvm/tools/clang": "https://llvm.googlesource.com/clang@release_38",
-  "third_party/externals/llvm/projects/compiler-rt": "https://llvm.googlesource.com/compiler-rt@release_38",
-  "third_party/externals/llvm/projects/libcxx": "https://llvm.googlesource.com/libcxx@release_38",
-  "third_party/externals/llvm/projects/libcxxabi": "https://llvm.googlesource.com/libcxxabi@release_38",
+deps_os = {
+  'llvm': {
+    # xSAN bots build Clang from scratch.
+    "third_party/externals/llvm": "https://llvm.googlesource.com/llvm@release_38",
+    "third_party/externals/llvm/tools/clang": "https://llvm.googlesource.com/clang@release_38",
+    "third_party/externals/llvm/projects/compiler-rt": "https://llvm.googlesource.com/compiler-rt@release_38",
+    "third_party/externals/llvm/projects/libcxx": "https://llvm.googlesource.com/libcxx@release_38",
+    "third_party/externals/llvm/projects/libcxxabi": "https://llvm.googlesource.com/libcxxabi@release_38",
+  }
 }
 
 recursedeps = [ "common" ]