Compile the uncompressing code with -fno-builtin

This is to avoid a compiler warning for overriding the built-in "putc"
function.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/boot/compressed/Makefile

@@ -75,7 +75,7 @@ SEDFLAGS	= s/TEXT_START/$(ZTEXTADDR)/;s/BSS_START/$(ZBSSADDR)/
 
 targets       := vmlinux vmlinux.lds piggy.gz piggy.o $(FONT) \
 		 head.o misc.o $(OBJS)
-EXTRA_CFLAGS  := -fpic
+EXTRA_CFLAGS  := -fpic -fno-builtin
 EXTRA_AFLAGS  :=
 
 # Supply ZRELADDR, INITRD_PHYS and PARAMS_PHYS to the decompressor via