kbuild: fix modkern_aflags implementation

For the single target building %.symtypes from %.S, $(a_flags) is
expanded into the _KERNEL flags even if the object is a part of a
module.

$(real-obj-m:.o=.symtypes): modkern_aflags := $(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE)

... would fix the issue, but it is not nice to duplicate similar code
for every suffix.

Implement modkern_aflags in the same way as modkern_cflags.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -309,10 +309,9 @@ $(obj)/%.h.s: $(src)/%.h FORCE
 # Compile assembler sources (.S)
 # ---------------------------------------------------------------------------
 
-modkern_aflags := $(KBUILD_AFLAGS_KERNEL) $(AFLAGS_KERNEL)
-
-$(real-obj-m)      : modkern_aflags := $(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE)
-$(real-obj-m:.o=.s): modkern_aflags := $(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE)
+modkern_aflags = $(if $(part-of-module),				\
+			$(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE),	\
+			$(KBUILD_AFLAGS_KERNEL) $(AFLAGS_KERNEL))
 
 # .S file exports must have their C prototypes defined in asm/asm-prototypes.h
 # or a file that it includes, in order to get versioned symbols. We build a