Split off AVX512 test from c_check to make its result available to getarch

Makefile.prebuild

@@ -71,10 +71,10 @@ endif
 
 
 getarch : getarch.c cpuid.S dummy $(CPUIDEMU)
-ifneq (,$(findstring NO_AVX512,$(file < $(TARGET_MAKE))))
-	$(HOSTCC) $(HOST_CFLAGS) $(EXFLAGS) -DNO_AVX512=1 -v -o $(@F) getarch.c cpuid.S $(CPUIDEMU) >getarchlog 2>&1
+ifeq (1, $(shell perl ./avx512_check $(CC) $(TARGET_FLAGS)))
+	$(HOSTCC) $(HOST_CFLAGS) $(EXFLAGS) -DNO_AVX512=1 -o $(@F) getarch.c cpuid.S $(CPUIDEMU)
 else
-	$(HOSTCC) $(HOST_CFLAGS) $(EXFLAGS) -v -o $(@F) getarch.c cpuid.S $(CPUIDEMU) >getarchlog 2>&1
+	$(HOSTCC) $(HOST_CFLAGS) $(EXFLAGS) -o $(@F) getarch.c cpuid.S $(CPUIDEMU)
 endif
 
 getarch_2nd : getarch_2nd.c config.h dummy