[build] Don't add to DOTNET_PLATFORMS if .NET isn't enabled (#18827)

This makes it easier to skip .NET stuff if .NET isn’t enabled.

Make.config

@@ -670,6 +670,7 @@ ALL_DOTNET_PLATFORMS=iOS macOS tvOS MacCatalyst
 
 DOTNET_PLATFORMS=
 ifdef INCLUDE_IOS
+ifdef ENABLE_DOTNET
 DOTNET_PLATFORMS+=iOS
 DOTNET_IOS_BITNESSES+=64
 
@@ -690,9 +691,11 @@ DOTNET_IOS_RUNTIME_IDENTIFIERS_64+=iossimulator-x64 iossimulator-arm64
 
 # All of them
 DOTNET_IOS_RUNTIME_IDENTIFIERS=$(DOTNET_IOS_RUNTIME_IDENTIFIERS_32) $(DOTNET_IOS_RUNTIME_IDENTIFIERS_64)
+endif # ENABLE_DOTNET
 endif # INCLUDE_IOS
 
 ifdef INCLUDE_TVOS
+ifdef ENABLE_DOTNET
 DOTNET_PLATFORMS+=tvOS
 DOTNET_TVOS_BITNESSES+=64
 ifdef INCLUDE_DEVICE
@@ -701,6 +704,7 @@ else
 DOTNET_TVOS_RUNTIME_IDENTIFIERS=tvossimulator-x64 tvossimulator-arm64
 endif
 DOTNET_TVOS_RUNTIME_IDENTIFIERS_64+=$(DOTNET_TVOS_RUNTIME_IDENTIFIERS)
+endif # ENABLE_DOTNET
 endif
 
 ifdef INCLUDE_DOTNET_WATCHOS
@@ -716,18 +720,22 @@ endif
 endif
 
 ifdef INCLUDE_MACCATALYST
+ifdef ENABLE_DOTNET
 DOTNET_PLATFORMS+=MacCatalyst
 DOTNET_MACCATALYST_BITNESSES+=64
 DOTNET_MACCATALYST_RUNTIME_IDENTIFIERS=maccatalyst-x64 maccatalyst-arm64
 DOTNET_MACCATALYST_RUNTIME_IDENTIFIERS_64+=$(DOTNET_MACCATALYST_RUNTIME_IDENTIFIERS)
+endif # ENABLE_DOTNET
 endif
 
 ifdef INCLUDE_MAC
+ifdef ENABLE_DOTNET
 DOTNET_PLATFORMS+=macOS
 DOTNET_CORECLR_PLATFORMS+=macOS
 DOTNET_MACOS_BITNESSES+=64
 DOTNET_MACOS_RUNTIME_IDENTIFIERS=osx-x64 osx-arm64
 DOTNET_MACOS_RUNTIME_IDENTIFIERS_64=$(DOTNET_MACOS_RUNTIME_IDENTIFIERS)
+endif # ENABLE_DOTNET
 endif
 
 ifdef INCLUDE_IOS