Probably if you compile – many times – Linux for embedded hardware you change Makefile to your specific architecture and compiler. An example:

# Set the ARCH and CROSS_COMPILE default values
ARCH            ?= arm
CROSS_COMPILE   ?= arm-unknown-linux-gnu-

Latest Linux (~2.6.31) turns it deprecated (commit 575543347b5baed0ca927cb90ba8807396fe9cc9). Now the settings are saved in two files named:

include/generated/kernel.arch
include/generated/kernel.cross

What changes?

Using the new way you don’t edit Makefile anymore, just need set once your definitions for cross compiling:

$ make ARCH=arm CROSS_COMPILE=arm-unknown-linux-gnu-

And for next builds just run make

$ make

This works both for plain builds and for O=… builds.