Add build option -fomit-frame-pointer.

It seems to improve code generation.
diff --git a/Makefile b/Makefile
index 50a29d5..6938bef 100644
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@
 
 # Default compiler flags
 COMMONCFLAGS = -Wall -Os -MD -m32 -march=i386 -mregparm=2 \
-               -ffreestanding -fwhole-program
+               -ffreestanding -fwhole-program -fomit-frame-pointer
 COMMONCFLAGS += $(call cc-option,$(CC),-nopie,)
 COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector,)
 COMMONCFLAGS += $(call cc-option,$(CC),-fno-stack-protector-all,)