[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.

lib/Makefile

@@ -212,11 +212,11 @@ $(ZSTD_STATLIB_DIR)/%.o : %.c $(ZSTD_STATLIB_DIR)/%.d | $(ZSTD_STATLIB_DIR)
 
 $(ZSTD_DYNLIB_DIR)/%.o : %.S | $(ZSTD_DYNLIB_DIR)
 	@echo AS $@
-	$(COMPILE.c) $(OUTPUT_OPTION) $<
+	$(COMPILE.S) $(OUTPUT_OPTION) $<
 
 $(ZSTD_STATLIB_DIR)/%.o : %.S | $(ZSTD_STATLIB_DIR)
 	@echo AS $@
-	$(COMPILE.c) $(OUTPUT_OPTION) $<
+	$(COMPILE.S) $(OUTPUT_OPTION) $<
 
 MKDIR ?= mkdir
 $(BUILD_DIR) $(ZSTD_DYNLIB_DIR) $(ZSTD_STATLIB_DIR):

lib/libzstd.mk

@@ -95,8 +95,9 @@ DEBUGFLAGS= -Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
             -Wvla -Wformat=2 -Winit-self -Wfloat-equal -Wwrite-strings \
             -Wredundant-decls -Wmissing-prototypes -Wc++-compat
 CFLAGS   += $(DEBUGFLAGS) $(MOREFLAGS)
+ASFLAGS  += $(DEBUGFLAGS) $(MOREFLAGS) $(CFLAGS)
 LDFLAGS  += $(MOREFLAGS)
-FLAGS     = $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)
+FLAGS     = $(CPPFLAGS) $(CFLAGS) $(ASFLAGS) $(LDFLAGS)
 
 ifndef ALREADY_APPENDED_NOEXECSTACK
 export ALREADY_APPENDED_NOEXECSTACK := 1
@@ -104,10 +105,12 @@ ifeq ($(shell echo "int main(int argc, char* argv[]) { (void)argc; (void)argv; r
 LDFLAGS += -z noexecstack
 endif
 ifeq ($(shell echo | $(CC) $(FLAGS) -Wa,--noexecstack -x assembler -Werror -c - -o $(VOID) 2>$(VOID) && echo 1 || echo 0),1)
-CFLAGS += -Wa,--noexecstack
+CFLAGS  += -Wa,--noexecstack
+# CFLAGS are also added to ASFLAGS
 else ifeq ($(shell echo | $(CC) $(FLAGS) -Qunused-arguments -Wa,--noexecstack -x assembler -Werror -c - -o $(VOID) 2>$(VOID) && echo 1 || echo 0),1)
 # See e.g.: https://github.com/android/ndk/issues/171
-CFLAGS += -Qunused-arguments -Wa,--noexecstack
+CFLAGS  += -Qunused-arguments -Wa,--noexecstack
+# CFLAGS are also added to ASFLAGS
 endif
 endif