Code review comment for ~mfo/ubuntu/+source/xen:lp1956166v2

FTBFS offending commit:

 /git/xen$ gl1 -1 --format=quote 7c9f81687ad6
 commit 7c9f81687ad6 ("introduce unaligned.h")

 /git/xen$ git describe --contains 7c9f81687ad6
 4.15.0-rc1~280

In upstream/4.15, lzo.c was no longer built on arm, just on x86,
but downstream/4.11, it still is.

 ~/git/xen$ gl1 -1 --format=quote c492e19fdd05
 commit c492e19fdd05 ("xen: remove tmem from hypervisor")

 ~/git/xen$ git show c492e19fdd05
 diff --git a/xen/common/Makefile b/xen/common/Makefile
 ...
 -obj-$(CONFIG_TMEM) += lzo.o
 ...
 -lzo-y := lzo
 -lzo-$(CONFIG_TMEM) :=
 -obj-bin-$(CONFIG_X86) += $(foreach n,decompress bunzip2 unxz unlzma $(lzo-y) unlzo unlz4 earlycpio,$(n).init.o)
 +obj-bin-$(CONFIG_X86) += $(foreach n,decompress bunzip2 unxz unlzma lzo unlzo unlz4 earlycpio,$(n).init.o)

 /git/xen$ git describe --contains c492e19fdd05
 4.13.0-rc1~972