Build: use CPP just for pre-processing

Using AS for pre-processing looks a bit weird, and some assembly
specific options are given for nothing.  Rather, use CPP.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -143,16 +143,14 @@ TF_CFLAGS_aarch64	=	-mgeneral-regs-only -mstrict-align
 ASFLAGS_aarch32		=	-march=armv8-a
 TF_CFLAGS_aarch32	=	-march=armv8-a
 
-ASFLAGS			+= 	-nostdinc -ffreestanding -Wa,--fatal-warnings	\
-				-Werror -Wmissing-include-dirs			\
-				-D__ASSEMBLY__ $(ASFLAGS_$(ARCH))		\
-				${DEFINES} ${INCLUDES}
-TF_CFLAGS		+= 	-nostdinc -ffreestanding -Wall			\
-				-Werror -Wmissing-include-dirs			\
-				-std=c99 -Os					\
-				$(TF_CFLAGS_$(ARCH))				\
-				${DEFINES} ${INCLUDES}
-TF_CFLAGS		+=	-ffunction-sections -fdata-sections
+CPPFLAGS		=	${DEFINES} ${INCLUDES} -nostdinc		\
+				-Wmissing-include-dirs -Werror
+ASFLAGS			+=	$(CPPFLAGS) $(ASFLAGS_$(ARCH))			\
+				-D__ASSEMBLY__ -ffreestanding 			\
+				-Wa,--fatal-warnings
+TF_CFLAGS		+=	$(CPPFLAGS) $(TF_CFLAGS_$(ARCH))		\
+				-ffreestanding -Wall -std=c99 -Os		\
+				-ffunction-sections -fdata-sections
 
 LDFLAGS			+=	--fatal-warnings -O1
 LDFLAGS			+=	--gc-sections

make_helpers/build_macros.mk

@@ -262,11 +262,11 @@ $(eval PREREQUISITES := $(1).d)
 
 $(1): $(2)
 	@echo "  PP      $$<"
-	$$(Q)$$(AS) $$(ASFLAGS) -P -E -D__LINKER__ -o $$@ $$<
+	$$(Q)$$(CPP) $$(CPPFLAGS) -P -D__ASSEMBLY__ -D__LINKER__ -o $$@ $$<
 
 $(PREREQUISITES): $(2) | $(dir ${1})
 	@echo "  DEPS    $$@"
-	$$(Q)$$(AS) $$(ASFLAGS) -M -MT $(1) -MF $$@ $$<
+	$$(Q)$$(CPP) $$(CPPFLAGS) -D__ASSEMBLY__ -M -MT $(1) -MF $$@ $$<
 
 ifdef IS_ANYTHING_TO_BUILD
 -include $(PREREQUISITES)