Use proper extension on the avx512 testcase filename

The need to call it .tmp existed only when it was generated by a tmpfile call, and the "-x c" option to tell the compiler it is actually a C source is not universally supported (this broke the test with clang-cl at least)

cmake/system_check.cmake

@@ -109,10 +109,10 @@ else()
 endif()
 
 if (X86_64 OR X86)
-  file(WRITE ${PROJECT_BINARY_DIR}/avx512.tmp "#include <immintrin.h>\n\nint main(void){ __asm__ volatile(\"vbroadcastss -4 * 4(%rsi), %zmm2\"); }")
-execute_process(COMMAND ${CMAKE_C_COMPILER} -march=skylake-avx512 -c -v -o ${PROJECT_BINARY_DIR}/avx512.o -x c ${PROJECT_BINARY_DIR}/avx512.tmp OUTPUT_QUIET ERROR_QUIET RESULT_VARIABLE NO_AVX512)
+  file(WRITE ${PROJECT_BINARY_DIR}/avx512.c "#include <immintrin.h>\n\nint main(void){ __asm__ volatile(\"vbroadcastss -4 * 4(%rsi), %zmm2\"); }")
+execute_process(COMMAND ${CMAKE_C_COMPILER} -march=skylake-avx512 -c -v -o ${PROJECT_BINARY_DIR}/avx512.o ${PROJECT_BINARY_DIR}/avx512.c OUTPUT_QUIET ERROR_QUIET RESULT_VARIABLE NO_AVX512)
 if (NO_AVX512 EQUAL 1)
 set (CCOMMON_OPT "${CCOMMON_OPT} -DNO_AVX512")
 endif()
-  file(REMOVE "avx512.tmp" "avx512.o")
+  file(REMOVE "avx512.c" "avx512.o")
 endif()