Move mac clangd build from x64 to arm (flutter/engine#56567)

Build and x64 dimension was introduced in flutter/engine#50901, and I blindly copied it into the .ci.yaml in flutter/engine#56014.  However I don't think clangd needs to run on the scarce (and flakier) Intel Macs (37 x64 bots in prod vs 55 arm bots).  Suspect this was copied from clang_tidy flutter#26910

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: jmagman

engine/src/flutter/.ci.yaml

@@ -509,7 +509,7 @@ targets:
     recipe: engine_v2/builder
     properties:
       config_name: mac_clangd
-      cpu: x86
+      cpu: arm64
 
   - name: Mac mac_unopt
     recipe: engine_v2/engine_v2

engine/src/flutter/ci/builders/standalone/mac_clangd.json

@@ -32,7 +32,7 @@
       "name": "clangd",
       "script": "flutter/tools/clangd_check/bin/main.dart",
       "parameters": [
-        "--clangd=buildtools/mac-x64/clang/bin/clangd",
+        "--clangd=buildtools/mac-arm64/clang/bin/clangd",
         "--compile-commands-dir=../out/ci/mac_unopt_debug_no_rbe"
       ]
     }