kbuild: use -fmacro-prefix-map for .S sources

Follow-up to commit a73619a8 ("kbuild: use -fmacro-prefix-map to
make __FILE__ a relative path"). Assembler sources also use __FILE__
macro so this flag should be also applied to those sources.
Signed-off-by: Denys Zagorui <dzagorui@cisco.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -946,7 +946,7 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=incompatible-pointer-types)
 KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 
 # change __FILE__ to the relative path from the srctree
-KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
+KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
 # ensure -fcf-protection is disabled when using retpoline as it is
 # incompatible with -mindirect-branch=thunk-extern