Ubuntu kernel 4.7 and 4.8 on ppc64el: Cross compilation issue

[Breno Leitao]

Hello kernel team,

I found a problem on Ubuntu kernel version 4.7 and 4.8 from kernel
mainline PPA (~kernel-ppa/mainline) that is happening on ppc64el
port.

I found that, starting at version 4.7, there is an issue with the
cross-compilation process, as showed:

	➜  ~ dpkg -S /usr/src/linux-headers-4.8.0-040800rc3-generic/scripts/dtc/dtc.o
	linux-headers-4.8.0-040800rc3-generic: /usr/src/linux-headers-4.8.0-040800rc3-generic/scripts/dtc/dtc.o
	➜  ~ file /usr/src/linux-headers-4.8.0-040800rc3-generic/scripts/dtc/dtc.o
	/usr/src/linux-headers-4.8.0-040800rc3-generic/scripts/dtc/dtc.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
	➜  ~ uname -a
	Linux 1610 4.8.0-040800rc3-generic #201608212032 SMP Mon Aug 22 01:21:51 UTC 2016 ppc64le ppc64le ppc64le GNU/Linux

Looking at the log, I see:

	-- dchroot -c yakkety-amd64 --directory=/home/kernel/COD/linux -- /bin/bash
	I: [yakkety-amd64-2bcf8cf2-911e-433c-805c-d10a6933743b chroot] Running command: “/bin/bash”
	dpkg-architecture: warning: specified GNU system type powerpc64le-linux-gnu does not match CC system type x86_64-linux-gnu, try setting a correct CC environment variable

Somehow, the dhcroot is using an amd64 other than a ppc64el image to get
things built.