[make] miscellaneous make system tweaks

-Have arch set the default optimization level. Most will use
 O2, but arm-m sets to Os.
-Make the link time garbage collection be optional. Only set
 for arm-m.

make/build.mk

@@ -1,3 +1,9 @@
+# use linker garbage collection, if requested
+ifeq ($(WITH_LINKER_GC),1)
+GLOBAL_COMPILEFLAGS += -ffunction-sections -fdata-sections
+GLOBAL_LDFLAGS += --gc-sections
+endif
+
 $(OUTBIN): $(OUTELF)
 	@echo generating image: $@
 	$(NOECHO)$(SIZE) $<