Fix aarch64 build, thanks @rsta2

https://github.com/probonopd/MiniDexed/pull/85#issuecomment-1095391183

src/Synth_Dexed.mk

@@ -34,6 +34,10 @@ INCLUDE += -I $(CMSIS_CORE_INCLUDE_DIR)
 INCLUDE += -I $(CMSIS_DSP_INCLUDE_DIR)
 INCLUDE += -I $(CMSIS_DSP_PRIVATE_INCLUDE_DIR)
 INCLUDE += -I $(CMSIS_DSP_COMPUTELIB_INCLUDE_DIR)
-DEFINE += -DARM_MATH_NEON -DHAVE_NEON
+
+DEFINE += -DARM_MATH_NEON
+ifeq ($(strip $(AARCH)),64)
+DEFINE += -DHAVE_NEON
+endif
 
 EXTRACLEAN = $(SYNTH_DEXED_DIR)/*.[od] $(CMSIS_DSP_SOURCE_DIR)/SupportFunctions/*.[od] $(CMSIS_DSP_SOURCE_DIR)/SupportFunctions/*.[od] $(CMSIS_DSP_SOURCE_DIR)/BasicMathFunctions/*.[od] $(CMSIS_DSP_SOURCE_DIR)/FastMathFunctions/*.[od] $(CMSIS_DSP_SOURCE_DIR)/FilteringFunctions/*.[od] $(CMSIS_DSP_SOURCE_DIR)/CommonTables/*.[od] $(CMSIS_DSP_COMPUTELIB_SRC_DIR)/*.[od]