Fix CMAKE_SYSTEM_PROCESSOR always hardcoded to ARM64

The script defined $CmakeArch correctly but never used it.
CMAKE_SYSTEM_PROCESSOR was always ARM64 even on x64 builds.

This was breaking x64 builds completely.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: WebKit ARM64 Builder

windows-release.ps1

@@ -269,7 +269,7 @@ if ($CcachePath) {
 
 cmake -S . -B $WebKitBuild `
     -DPORT="JSCOnly" `
-    -DCMAKE_SYSTEM_PROCESSOR=ARM64 `
+    "-DCMAKE_SYSTEM_PROCESSOR=${CmakeArch}" `
     @CcacheLauncher `
     -DENABLE_STATIC_JSC=ON `
     -DALLOW_LINE_AND_COLUMN_NUMBER_IN_BUILTINS=ON `