ARM: turn on link time optimisation (LTO).

Suggested by @Wurstnase. Apparently gcc got better, so it's
actually an advantage now.

Actually a pretty big advantage. While binary size decreases some
200 bytes, pulse length of the debug LED is a lot shorter
(measured on the scope):

  without LTO:  4.59 us
  with LTO:     3.65 us

That's a 25% performance increase by just turning on a flag!

Makefile-ARM

@@ -107,7 +107,7 @@ endif
 CFLAGS += -Wall
 CFLAGS += -Wstrict-prototypes
 CFLAGS += -Os
-#CFLAGS += -flto
+CFLAGS += -flto
 CFLAGS += -ffunction-sections
 CFLAGS += -fdata-sections
 #CFLAGS += -dM -E # To list all predefined macros into the .o file.