Allow linking libruby to libc++abi on amd64/aarch64, so other
software that links to libruby other than ruby itself does
not need to manually handle arch-dependent linking.

Index: template/Makefile.in
--- template/Makefile.in.orig
+++ template/Makefile.in
@@ -339,7 +339,7 @@ verify-static-library: $(LIBRUBY_A)
 $(LIBRUBY_SO):
 		@-[ -n "$(EXTSTATIC)" ] || $(PRE_LIBRUBY_UPDATE)
 		$(ECHO) linking shared-library $@
-		$(Q) $(LDSHARED) $(DLDFLAGS) $(OBJS) $(DLDOBJS) $(SOLIBS) $(EXTSOLIBS) $(OUTFLAG)$@
+		$(Q) $(LDSHARED) ${LIBRUBY_DLDFLAGS} $(DLDFLAGS) $(OBJS) $(DLDOBJS) $(SOLIBS) $(EXTSOLIBS) $(OUTFLAG)$@
 		-$(Q) $(OBJCOPY) -w -L '$(SYMBOL_PREFIX)Init_*' -L '$(SYMBOL_PREFIX)ruby_static_id_*' \
 			-L '$(SYMBOL_PREFIX)*_threadptr_*' -L '$(SYMBOL_PREFIX)*_ec_*' $@
 		$(Q) $(POSTLINK)