VS: Add ARM64EC to supported platforms for VS 16 and 17

In commit 4ea3a88625 (MSVC: Add support for targeting ARM64EC,
2020-12-30, v3.20.0-rc1~121^2) the `ARM64EC` platform was accidentally
added to the list for VS 15 (2017) instead of VS 16 (2019).  Its
omission from the list of platforms was then repeated for VS 17 (2022).

Issue: #21724

Source/cmGlobalVisualStudioVersionedGenerator.cxx

@@ -211,7 +211,6 @@ public:
     platforms.emplace_back("Win32");
     platforms.emplace_back("ARM");
     platforms.emplace_back("ARM64");
-    platforms.emplace_back("ARM64EC");
     return platforms;
   }
 
@@ -305,6 +304,7 @@ public:
     platforms.emplace_back("Win32");
     platforms.emplace_back("ARM");
     platforms.emplace_back("ARM64");
+    platforms.emplace_back("ARM64EC");
     return platforms;
   }
 
@@ -369,6 +369,7 @@ public:
     platforms.emplace_back("Win32");
     platforms.emplace_back("ARM");
     platforms.emplace_back("ARM64");
+    platforms.emplace_back("ARM64EC");
     return platforms;
   }