mac: Remove debug option fro MJIT_DEBUGFLAGS

Not to generate .dSYM directories by MJIT runs, which are left in
large numbers after each test.

defs/gmake.mk

@@ -11,7 +11,10 @@ override ACTIONS_GROUP = @echo "\#\#[group]$(patsubst yes-%,%,$@)"
 override ACTIONS_ENDGROUP = @echo "\#\#[endgroup]"
 endif
 
-ifneq ($(filter %darwin%,$(arch)),)
+ifneq ($(filter darwin%,$(target_os)),)
+# Remove debug option not to generate thousands of .dSYM
+MJIT_DEBUGFLAGS := $(filter-out -g%,$(MJIT_DEBUGFLAGS))
+
 INSTRUBY_ENV += SDKROOT=/
 endif
 INSTRUBY_ARGS += --gnumake