build: Automatically include architecture-specific library Makefile snippets.

Makefile

@@ -112,6 +112,7 @@ define DOSUBDIR
 $(foreach V,$(SUBDIR_VARS),$(eval $(call RESET,$(V))))
 SUBDIR := $(1)/
 include $(SRC_PATH)/$(1)/Makefile
+-include $(SRC_PATH)/$(1)/$(ARCH)/Makefile
 include $(SRC_PATH)/library.mak
 endef
 

libavcodec/Makefile

@@ -670,8 +670,6 @@ OBJS-$(CONFIG_MLIB)                    += mlib/dsputil_mlib.o           \
 # well.
 OBJS-$(!CONFIG_SMALL)                  += inverse.o
 
--include $(SRC_PATH)/$(SUBDIR)$(ARCH)/Makefile
-
 SKIPHEADERS                            += %_tablegen.h                  \
                                           %_tables.h                    \
                                           aac_tablegen_decl.h           \

libavfilter/Makefile

@@ -69,8 +69,6 @@ OBJS-$(CONFIG_TESTSRC_FILTER)                += vsrc_testsrc.o
 
 OBJS-$(CONFIG_NULLSINK_FILTER)               += vsink_nullsink.o
 
--include $(SRC_PATH)/$(SUBDIR)$(ARCH)/Makefile
-
 DIRS = x86
 
 TOOLS = graph2dot lavfi-showfiltfmts