build: Drop arch-specific checkasm Makefiles

They only contain one line and will never contain more.

tests/checkasm/Makefile

@@ -19,7 +19,9 @@ AVCODECOBJS-$(CONFIG_VP9_DECODER)       += vp9dsp.o
 CHECKASMOBJS-$(CONFIG_AVCODEC)          += $(AVCODECOBJS-yes)
 
 
--include $(SRC_PATH)/tests/checkasm/$(ARCH)/Makefile
+CHECKASMOBJS-$(ARCH_AARCH64)            += aarch64/checkasm.o
+CHECKASMOBJS-$(HAVE_ARMV5TE_EXTERNAL)   += arm/checkasm.o
+CHECKASMOBJS-$(HAVE_YASM)               += x86/checkasm.o
 
 CHECKASMOBJS += $(CHECKASMOBJS-yes) checkasm.o
 CHECKASMOBJS := $(sort $(CHECKASMOBJS:%=tests/checkasm/%))

tests/checkasm/aarch64/Makefile

-CHECKASMOBJS += aarch64/checkasm.o

tests/checkasm/arm/Makefile

-CHECKASMOBJS-$(HAVE_ARMV5TE_EXTERNAL) += arm/checkasm.o

tests/checkasm/x86/Makefile

-CHECKASMOBJS-$(HAVE_YASM) += x86/checkasm.o