stm32: stop pretending startup asm is C++

CMake 3.20+ interpret the LANGUAGE property more strictly and try to compile the file as C++ instead of just using the C++ compiler to compile it.

32blit-stm32/CMakeLists.txt

diff --git a/32blit-stm32/CMakeLists.txt b/32blit-stm32/CMakeLists.txt
index 33b1c09..bbab4b0 100644
--- a/32blit-stm32/CMakeLists.txt
+++ b/32blit-stm32/CMakeLists.txt
@@ -1,3 +1,5 @@
+enable_language(ASM)
+
 add_subdirectory(../32blit-shared ../32blit-shared)
 
 add_library(BlitHalSTM32 OBJECT
@@ -107,8 +109,6 @@
 )
 
 set_source_files_properties(
-	startup_stm32h750xx.s
-
 	Src/main.c
   	Src/usbd_msc_scsi.c
 	Src/usb_device.c

32blit-stm32/executable.cmake

diff --git a/32blit-stm32/executable.cmake b/32blit-stm32/executable.cmake
index aee239e..d354838 100644
--- a/32blit-stm32/executable.cmake
+++ b/32blit-stm32/executable.cmake
@@ -1,3 +1,5 @@
+enable_language(ASM)
+
 set(MCU_LINKER_SCRIPT "${CMAKE_CURRENT_LIST_DIR}/${MCU_LINKER_SCRIPT}")
 set(HAL_DIR ${CMAKE_CURRENT_LIST_DIR})
 
@@ -28,7 +30,6 @@
 		return()
 	endif()
 
-	set_source_files_properties(${USER_STARTUP} PROPERTIES LANGUAGE CXX)
 	add_executable(${NAME} ${USER_STARTUP} ${SOURCES})
 
 	# Ideally we want the .blit filename to match the .elf, but TARGET_FILE_BASE_NAME isn't always available