[nostalgia,turbine] Make GBA files (other than ARM asm) build on all platforms

src/turbine/gba/CMakeLists.txt

@@ -1,16 +1,33 @@
-enable_language(CXX ASM)
-set_source_files_properties(turbine.arm.cpp irq.arm.cpp PROPERTIES COMPILE_FLAGS -marm)
+if(TURBINE_BUILD_TYPE STREQUAL "GBA")
+	enable_language(CXX ASM)
+endif()
+
+add_library(Turbine-GBA OBJECT)
 target_sources(
-	Turbine PRIVATE
+	Turbine-GBA PRIVATE
 		clipboard.cpp
 		gfx.cpp
 		irq.arm.cpp
-		irq.s
 		turbine.arm.cpp
 		turbine.cpp
 )
 
+if(TURBINE_BUILD_TYPE STREQUAL "GBA")
+	set_source_files_properties(turbine.arm.cpp irq.arm.cpp PROPERTIES COMPILE_FLAGS -marm)
+	target_sources(
+		Turbine-GBA PRIVATE
+			irq.s
+	)
+	target_link_libraries(Turbine PUBLIC Turbine-GBA)
+else()
+	target_sources(
+		Turbine-GBA PRIVATE
+			irqstub.cpp
+	)
+endif()
+
 target_link_libraries(
-	Turbine PUBLIC
+	Turbine-GBA PUBLIC
 		TeaGBA
+		Keel
 )