stm32/Makefile: Use -O2 to optimise compilation of lib/libc/string0.c.

ports/stm32/Makefile

@@ -350,6 +350,11 @@ OBJ += $(addprefix $(BUILD)/, $(SRC_USBDEV:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_MOD:.c=.o))
 OBJ += $(BUILD)/pins_$(BOARD).o
 
+# This file contains performance critical functions so turn up the optimisation
+# level.  It doesn't add much to the code size and improves performance a bit.
+# Don't use -O3 with this file because gcc tries to optimise memset in terms of itself.
+$(BUILD)/lib/libc/string0.o: COPT += -O2
+
 # We put several files into the first 16K section with the ISRs.
 # If we compile these using -O0 then it won't fit. So if you really want these
 # to be compiled with -O0, then edit boards/common.ld (in the .isr_vector section)