build: enable -ffast-math.

By default, GCC implements strict IEEE 754 confirmance and chooses
bitwise accuracy over speed.  -ffast-math relaxes this and lets GCC do
optimisations like re-writing a / constant as a * (1 / constant).

Saves 144 bytes of flash and should be a speedup.

Signed-off-by: Michael Hope <mlhx@google.com>

Makefile

@@ -564,6 +564,7 @@ CFLAGS		 = $(ARCH_FLAGS) \
 		   $(DEBUG_FLAGS) \
 		   -std=gnu99 \
 		   -Wall -Wextra -Wunsafe-loop-optimizations -Wdouble-promotion \
+		   -ffast-math \
 		   -ffunction-sections \
 		   -fdata-sections \
 		   $(DEVICE_FLAGS) \