[build][asm] Pass ASFLAGS to the assembler instead of CFLAGS

* Add `-Wa,--noexecstack` to both `ASFLAGS` and `CFLAGS`
* Pass `ASFLAGS` to `.S` compilation instead of `CFLAGS`

Fixes #3006.

programs/Makefile

@@ -320,7 +320,7 @@ $(BUILD_DIR)/%.o : %.c $(BUILD_DIR)/%.d | $(BUILD_DIR)
 
 $(BUILD_DIR)/%.o : %.S | $(BUILD_DIR)
 	@echo AS $@
-	$(COMPILE.c) $(OUTPUT_OPTION) $<
+	$(COMPILE.S) $(OUTPUT_OPTION) $<
 
 MKDIR ?= mkdir
 $(BUILD_DIR): ; $(MKDIR) -p $@